Most animals in the world are invertebrates. If most invertebrates are sentient, then we can conclude that most of the suffering of animals in the wild is endured by invertebrates. To add to this, most of the animals used by humans in harmful ways are invertebrates.
For these reasons, invertebrate sentience is a very important issue. However, it is currently not possible to know for certain which of these animals are sentient, because that would require knowing the way in which consciousness occurs. If we understood how consciousness arises, we could identify which structures are capable of making consciousness possible. But we are still very far from having such knowledge, and it seems likely that we won’t know for at least a few decades.
What we have at this point are different indicators that help us estimate if it is more or less likely that invertebrates belonging to certain phyla or classes of animals are sentient. Those indicators can be of very different kinds. Some of the most important ones have to do with the structure of their nervous systems. These are not the only indicators — others such as behaviors that suggest learning and remembering can be very illuminating as well. Here, however, we focus on factors related to the structure and organization of the nervous systems of invertebrates.
The factors that we consider indicators of consciousness include the level of centralization of the nervous systems and the presence of related structures. The most important of these may be centralized structures that process information in a way that is needed for sentience to arise. In addition to this, other information could be useful, such as the size of the nervous systems, although it seems that the processing of information made possible by some form of centralization may be more important.
This article considers the relevant features of the nervous systems of different invertebrates with regard to these criteria. We have classified them according to certain phyla or classes of invertebrates in a way that reflects the huge diversity among these animals. The information below includes some schematic illustrations that are meant not to reproduce these animals’ nervous systems in detail, but to provide an idea of how centralization occurs in them.
Jellyfish, sea anemones, coral
The nervous system consists of one or more nerve nets. Information is likely to be integrated in the sensory ganglia.1 These animals possess a central nerve net rather than a body concentrating neurons.
Diffuse and/or through-conduction nerve nets.
Seems that among invertebrates with nervous systems, these have some of the most decentralized ones.
The nervous system size may be of approximately 5,000-20,000 nerve cells in a jellyfish.3
Some of the less mobile cnidaria contain diffuse nerve nets, which transmit information bi-directionally (signals can travel in either direction through the network). Some more motile forms also contain a through-conduction net which transmits information unidirectionally and faster. Some cnidaria, such as sea anemones, contain both types of component in a single nerve-net. Integrative activity in cnidaria might occur in the sensory ganglia, which may represent the first steps toward a centralized nervous system.4
The radial nature of the jellyfish anatomy means cephalization is not possible because all regions of the circumference are more or less equally responsive to environmental perturbations. Accordingly, it seems we cannot say that cnidaria have a central nervous system. However, an argument has been made that there is some centralization of structure that interacts with the more diffuse nerve nets in the rest of the system.5
Starfish, sea urchins, sea cucumbers
These animals have a circumoral nerve ring, which may be analogous to a brain. However, it may be that most sensory input is integrated peripherally and that there is no need for a central brain structure.
Ectoneural subsystem, containing the circumoral nerve ring and outer part of the radial nerve cords. Includes sensory and motor components.
Hyponeural subsystem, a thinner, inner layer of the radial nerve cords thought to control locomotion.
As in cnidaria, these are very decentralized nervous systems that do not contain a centralized concentration of neurons.
The information in the literature about these animals’ nervous systems is not very large in comparison to that for other animals.
The basic nervous system structure of echinodermata has traditionally been described as containing two separate compartments, the ectoneural and hyponeural subsystems. The ectoneural subsystem comprises the circumoral nerve ring and the thicker, outer part of the radial nerve cords, and has been ascribed both sensory and motor components. The hyponeural subsystem is a thinner, inner layer of the radial nerve cords thought to control locomotion. These subsystems are connected by short neural bridges.6
Mashanov et al. (2009) refer to the circumoral nerve ring and (usually) five radial nerve cords in echinoderms as a centralized nervous system.7 This circumoral nerve would thus coordinate body-wide behaviors and functions. However, Díaz-Balzac & García-Arrarás (2018) claim that there is little evidence to support this centralization, and a more plausible scenario would be that in echinoderms there is no “real” coordination center, and that nervous system functions and control are more widely dispersed throughout the body than in most other animals.8 They point out that the data available at present are largely anatomical in nature and so the extent to which echinoderm nervous systems employ peripheral or central integration of sensory input and motor output is largely unknown. The radially organized nervous system may use extensive peripheral integration and have no need for a structure equivalent to a brain.
These animals have highly centralized nervous systems, with most neurons concentrated in several anterior ganglia. There is a distinct brain region which forms a ring shape.
Circumoral nerve ring. Anterior ganglia.
The C. elegans connectome,2 like the brains of vertebrates such as mammals, features highly connected hubs, which are themselves interconnected in a central core structure. Thus, the macroscopic organization of the C. elegans nervous system shows scale-invariant conservation with the brains of vertebrates over many orders of magnitude of anatomical complexity.
The nervous system size of these animals is of approximately 102 to 103 neurons. C. elegans contains 302 neurons.9 Anguilla aceti contains 279 nerve cells and Ascaris lumbricoides contains 254 neurons.10
Nematodes have a high degree of centralization, with three-quarters of all nerve cells concentrated in a group of anteriorly placed ganglia and no peripheral plexuses or nets. They usually have eight longitudinal cords, commissures11 between dorsal and ventral cords, six cephalic nerves, a few special ganglia and nerves in the tail, and two sympathetic systems (one anterior and one posterior).12
The central nervous system of these animals consists primarily of a circumoral brain. Its ganglia are unlike the ganglia of other animals in that they do not contain neuropil, only cell bodies (the neuropil is any area in the nervous system composed of mostly unmyelinated axons, dendrites and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies). All nematodes have a major nerve cord running longitudinally along the ventral midline of the body, from head to tail. This ventral nerve cord contains cell bodies as well as processes, many of which project into the nerve ring where they make and receive synapses with other neurons.
The C. elegans connectome, like the human brain, also features highly connected hubs, which are themselves interconnected in a central core structure. These core structures are thought to be important for long-range communication and for linking different brain modules. Interestingly, the hubs in C. elegans are single neurons, while those in the human brain are cortical areas composed of many millions of neurons. Thus, the macroscopic organization of the C. elegans nervous system shows scale-invariant conservation with the human brain over many orders of magnitude of anatomical complexity.13
Key features include a circumoral brain or nerve ring, composed of axonal and dendritic processes, clusters of cephalic neuronal cell bodies, a ventral nerve cord containing processes and motor neuron cell bodies, and a dorsal nerve cord and circumferential commissures consisting exclusively of processes.
They have a centralized nervous system with a distinct brain located in the head. This brain receives and integrates information from sensory structures across the whole body.
Centralized brain structure located in the head.
Continuous EEG waveforms have been recorded from the planarian brain. The continuous waveforms suggest the existence of feedback loop circuits in the neural network. This continuous waveform is similar to that recorded from more developed brains.
These animals have a central nervous system with a distinct brain. Some groups, including the parasitic forms, are neurologically very simple, while others such as planarians are more complex. Many major sensory structures are found in the head since it is the region of the body that first samples the environment during forward locomotion. The concentration of sensory receptors has led to a concentration of neurons in the head region (cephalization), resulting in a centralized brain structure. This neuron mass receives and integrates signals from sensory structures located both on the head and on other parts of the body. At least six types of superficial sense organs are differentiated. The brain also likely directs and controls the behavior of many effectors.14
Continuous EEG waveforms have been recorded from planarian brains and these signals were modulated by intense light, anaesthesia, and vibration. The continuous waveforms suggest the existence of feedback loop circuits in the neural network. The continuous waveform of the EEG is similar to that of evolutionarily advanced animals with more developed brains.15
Some of these animals have a centralized nervous system with a distinct brain. Some taxa contain higher brain structures such as mushroom bodies (also found in arthropods). However, most annelids lack these brain structures. The standard annelid brain is ring-shaped with two cerebral ganglia.
Two cerebral ganglia forming a bilobed brain (in most annelids). Mushroom bodies and glomerular neuropil (in some annelids).
Some annelid brains can be divided into fore, mid and hind sections (this mirrors the gross structure of advanced brains such as the human brain).
Some taxa contain mushroom bodies, which are considered higher brain centers in insects.
Leeches are reported to have about 400 neurons in each of the 21 ganglia which make up their central nervous system, giving an estimated total of 8400 neurons in their entire central nervous system.16
Annelids contain a central nervous system and a distinct brain. The brain cannot be divided into fore, mid and hind segments because these divisions are difficult or impossible to distinguish in most annelids, particularly the sedentary species17 (although in predatory types such aspolychaetes, which contain more complicated brains, these may be visible18).
Several taxa contain higher brain centers such as mushroom bodies and glomerular neuropil (suggesting similar morphology to arthropods); however, these brain centers are missing in sedentary species of annelid. For example, the ring-shaped brain of the annelid “Oweniidae” is described as lacking ganglia, higher brain centers, or complex sensory organs.19 Although some more basic forms of annelid might not contain ganglia, it should be noted that the standard earthworm brain contains two cerebral ganglia that form a bilobed brain, which is connected to other parts of the body by sensory and motor nerve fibers.20
Clams, oysters, mussels
These animals’ nervous systems have some level of centralization, as they include 3 pairs of ganglia connected by a nerve cord.
Cerebropleural ganglia, visceral ganglia, pedal ganglia.
Unlike in other molluscs, there is no distinctive brain structure, although there is centralization in ganglia.
It has been estimated that a small clam has about 6000 neurons.21
The nervous system in bivalves is far less centralized than in many other molluscs. A pair of cerebropleural ganglia lie near the mouth and a pair of nerve cords connect them to the visceral ganglia posteriorly and the pedal ganglia anteriorly. There is no cephalic structure recognizable as a differentiated brain.22 Despite this, there is some centralization in their ganglia.
Snails and slugs
Typically contain 5 pairs of ganglia, including the cerebral ganglia, which are structurally and functionally differentiated and receive and send signals across the body.
Cerebral ganglia (in head). Procerebrum (in cerebral ganglia), which may contain the learning mechanism.
5 paired ganglia throughout body (typically).
These nervous systems are similar to those of bivalves, although a bit bigger and with ganglia serving as a brain.
These animals may have nervous systems with approximately 104 to 105 neurons.23 The nervous systems of opisthobranchs may contain about 5,000 neurons.24 The cerebral ganglia of the land slug Aplysia californica may contain approximately 2,400 neurons.25
In snails, a pair of cerebral ganglia constitute the brain. Typically, gastropods contain 5 distinct ganglia. These are connected longitudinally by nerve cords and laterally by commissures. A study found the cerebral ganglion of a snail to be functionally and structurally differentiated. It receives convergent sensory inputs from a variety of anterior sensory organs and the posterior body wall. Its outputs include motor commands to muscles and premotor commands towards executory centers in other ganglia. The procerebrum is a differentiated region of the cerebral ganglion and is speculated to contain a learning mechanism.26
Bees, fruit flies, grasshoppers
They have a centralized nervous system with a distinct brain. The insect brain is segmented into three main regions. One of these regions, the protocerebrum, contains the mushroom bodies which contain a large proportion of the overall neurons in the central nervous system. There is some decentralization – for example the ventral nerve cord is able to direct complex action even when the central brain complex has been completely disconnected.
Mushroom bodies, which are important for learning, memory and integrating information.
There is decentralization to a greater extent than exists among vertebrate organisms. This is evidenced by the retention of some complex behavior even when they suffer severe injuries in their head.
Neural correlates of attention have been recorded in Drosophila.
The Drosophila connectome shows a small-world organization (high connectivity between neighboring regions along with “short-cut” connections to distant regions) comparable to fiber tract networks found in certain mammals.
The nervous systems of these animals may have approximately 105 to 106 neurons in most insects.27 Drosophila are at the low end, containing approximately 105, while honeybees may have 106.28 The smallest known number of neurons in any insect is found in the parasitic wasp Megaphragma, which have around 7,400 neurons in their central nervous system.29
The insect brain is made up of three regions, the protocerebrum, deuterocerebrum and tritocerebrum. The protocerebrum is the largest and contains the mushroom bodies, which are higher brain centers that are important in learning and memory and in the control of complex behavior. These mushroom bodies receive and integrate distinct multisensory information in many segregated input layers.30 Approximately one third of the neurons in the honeybee are located in the mushroom bodies.31
The insect nervous system is somewhat decentralized. For example, the ventral nerve cord is responsible for some outputs including movements and mating — meaning that insects can retain a surprising degree of function when decapitated.
The drosophila (fruit fly) connectome is comparable to fiber tract networks among macaques, as it also shows a small-world organization (high connectivity between neighboring regions along with “short-cut” connections to distant regions) despite different brain size and architecture.32
Local field potentials were recorded in Drosophila and a 20-30 Hz response was recorded in the brain which was modulated by salience. This constitutes a physiological signature of object salience in the fruit fly.33
Crabs, lobsters, woodlice
Contains a central nervous system. The largest ganglion in it is found at the anterior end and functions as the brain.
Anterior ganglion (the brain).
Crustacean brains are somewhere between insects and cephalopods in terms of size and complexity.
Crustaceans have nervous systems that can be relatively large: the brain of large crustaceans such as lobsters is likely to be considerably larger than that of some vertebrates.34 The size and complexity of the brain lies somewhere between those of insects and octopuses. A crayfish may have about 10,000 neurons in their brain.35 The central nervous system comprises a double ventral nerve cord linking a series of ganglia. The largest ganglion is found at the anterior end and functions as the brain.
Have a centralized nervous system with a central brain structure. However, only approximately 1/10 of an octopus’ neurons are found in this central structure.
It should be noted that of the three main designs of cephalopod nervous system, the nautilus system is simpler than the decapod and octopod (although still more complex than any non-cephalopod mollusc).
The central brain structure (approx. 40 million neurons in octopuses). Particularly the vertical lobe, which contains around 25 million of these neurons and is involved in learning and memory.
Optic lobes (approx. 120-180 million neurons in octopi).
Tentacles (approx. 300 million neurons in octopi). Appear to retain a significant amount of function without connection to central brain.
Evidence of greater decentralization than in vertebrates, since individual tentacles appear to contain the necessary neural circuitry for voluntary movement.
The brains of octopuses produce similar EEG recordings to vertebrates.
Analogies have been drawn between lobes in the cephalopod brain and the thalamus in vertebrates, which likely plays a role in pain and consciousness.
Analogies have been drawn between the vertical lobe in the octopus and mushroom bodies in insects.
Octopuses have nervous systems that can contain approximately 500 million cells (the largest and most complex nervous system of the cephalopods). However, only about 45 million of these cells are in the central brain.36
The size of the cephalopod nervous system relative to body weight lies within the same range as vertebrate nervous systems (smaller than birds and mammals but larger than fish and reptiles). The two major concentrations of neurons in octopuses, the optic lobes and the nervous system of the arms, lie outside the central brain. The optic lobes contain 120 to 180 million neurons. The arm nervous system contains 300 million of the 500 million neurons in an octopus’ nervous system. The tentacles of an octopus can function somewhat independently, and severed tentacles have been shown to be controllable by mechanical or electrical stimulation, suggesting that the basic motor program for voluntary movement is embedded within the neural circuitry of the tentacle itself.37
Budelmann writes that the cephalopod nervous system is the most complex of any invertebrate nervous system. The cephalopod nervous system also has the highest degree of centralization of any mollusc and is the most centralized of any invertebrate besides insects. In cephalopods the degree of centralization corresponds well with the level of behavioral complexity of the animal. There are three d structures, the nautilus nervous system is simpler and lacks the higher brain centers for learning and memory.38 However, even nautilus has “vastly more channels and complex parts than any non-cephalopod mollusc.”39
The cephalopod brain is anatomically molluscan, making attempts to draw parallels between more than 30 lobes identified in its central nervous system and the brains of vertebrate species unrealistic. However, there are some clear resemblances, e.g. the neural-architecture of the peduncle lobe in the octopus brain is similar to the folia arrangements of the vertebrate cerebellum, and the vertical lobe is considered the analogue of the mammalian limbic lobe. It has been suggested that in the cephalopods dorsal basal- and sub-vertical lobes could be considered as candidates for analogues to the vertebrate thalamus (which has been implicated in the experience of pain and consciousness).40
The vertical lobe is an important part of the nervous systems of cephalopods, and is implicated in learning and memory. It serves a function analogous to the mushroom bodies in insect brains. It is also an area of high neuron density, containing around 25 million of the 40-45 million neurons in the octopus brain.41
Octopus brains show similar EEG to vertebrates. Event-related potentials have been recorded in octopuses, which are EEG signals that are associated with cognitive events in humans.42
As we have seen, despite the great differences among these animals and their nervous systems, we often find centralization to different degrees. Because centralized nervous systems are an indicator of sentience, the available evidence about the structure and function of the nervous systems of invertebrates suggests that many of them are plausible candidates for sentience. In some cases this is very clear, as when they have relatively complex nervous systems like cephalopods. In other cases it may be less certain but sentience cannot be ruled out, as in bivalves. We don’t currently have the means to solve these problems, because the physiological basis of consciousness remains unknown. But this overview of invertebrate nervous systems shows that sentience might arise in invertebrates from many different types of nervous systems.
For more information, see our literature review on the neuroscientific evidence for invertebrate sentience.
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1 A ganglion is a nerve cell cluster or a group of nerve cell bodies located in the autonomic nervous system and sensory system.
2 A connectome is a comprehensive map of neural connections in the brain (wiring diagram): “More broadly, a connectome would include the mapping of all neural connections within an organism’s nervous system.”
3 Bode, H.; Berking, S.; David, C. N.; Gierer, A.; Schaller, H. & Trenkner, E. (1973) “Quantitative analysis of cell types during growth and morphogenesis in Hydra”, op. cit., p. 7. Garm, A.; Poussart, Y.; Parkefelt, L.; Ekström, P. & Nilsson, D.-E. (2007) “The ring nerve of the box jellyfish Tripedalia cystophora”, Cell and Tissue Research, 329, pp. 147-157.
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11 Commissures are fiber tracts that connect the two cerebral hemispheres in the brain and span the longitudinal fissure.
12 Erulkar, S. D. & Lentz, T. L. (2004 ) “Nervous system: Anatomy”, op. cit.
14 Bullock, T. H. (1977) Introduction to nervous systems, op. cit. Usherwood, P. N. R. (1973) Nervous systems, London: Edward Arnold.
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18 Erulkar, S. D. & Lentz, T. L. (2004 ) “Nervous system: Anatomy”, op. cit.
19 Beckers, P.; Helm, C.; Purschke, G.; Worsaae, K.; Hutchings, P. & Bartolomaeus, T. (2019) “The central nervous system of Oweniidae (Annelida) and its implications for the structure of the ancestral annelid brain”, op. cit.
20 Erulkar, S. D. & Lentz, T. L. (2004 ) “Nervous system: Anatomy”, op. cit.
22 Thorp, J. H. (2010 ) Ecology and classification of North American freshwater invertebrates, op. cit., p. 326.
23 Gelperin, A. & Tank, D. W. (1990) “Odour-modulated collective network oscillations of olfactory interneurons in a terrestrial mollusc”, Nature, 345, pp. 437-440.
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25 Cash, D. & Carew, T. J. (1989) “A quantitative analysis of the development of the central nervous system in juvenile Aplysia californica”, Journal of Neurobiology, 20, pp. 25-47.
26 Chase, R. (2000) “Structure and function in the cerebral ganglion”, Microscopy Research & Technique, 49, pp. 511-520.
27 Eisemann, C. H.; Jorgensen, W. K.; Merritt, D. J.; Rice, M. J.; Cribb, B. W.; Webb, P. D. & Zalucki, M. P. (1984) “Do insects feel pain? — A biological view”, Experientia, 40, pp. 164-167.
28 Hadley, D. (2019) “Do insect have brains?”, op. cit.
29 Polilov, A. A. (2012) “The smallest insects evolve anucleate neurons”, Arthropod Structure & Development, 41, pp. 29-34.
30 Gronenberg, W. & López-Riquelme, G. O. (2004) “Multisensory convergence in the mushroom bodies of ants and bees”, Acta Biologica Hungarica, 55, pp. 31-37.
31 Hadley, D. (2019) “Do insect have brains?”, op. cit.
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33 Swinderen, B. van & Greenspan, R. J. (2003) “Salience modulates 20-30 Hz brain activity in Drosophila”, Nature Neuroscience, 6, pp. 579-586.
34 Elwood, R. W.; Barr, S. & Patterson, L. (2009) “Pain and stress in crustaceans?”, Applied Animal Behaviour Science, 118, pp. 128-136.
35 Wiersma, C. (1957) “On the number of nerve cells in a crustacean central nervous system”, Acta Physiologica et Pharmacologica Neerlandica, 6, 135-142.
36 Hochner, B.; Shomrat, T. & Fiorito, G. (2006) “The octopus: A model for a comparative analysis of the evolution of learning and memory mechanisms”, The Biological Bulletin, 210, pp. 308-317 [accessed on 15 January 2021].
37 Sumbre, G.; Gutfreund, Y.; Fiorito, G.; Flash, T. & Hochner, B. (2001) “Control of octopus arm extension by a peripheral motor program”, Science, 293, pp. 1845-1848. Hochner, B.; Shomrat, T. & Fiorito, G. (2006) “The octopus: A model for a comparative analysis of the evolution of learning and memory mechanisms”, op. cit. Young, J. Z. (1963) “The number and sizes of nerve cells in octopus”, Proceedings of the Zoological Society of London, 140, pp. 229-254.
38 Budelmann, B. U. (1995) “The cephalopod nervous system: What evolution has made of the molluscan design”, in Breidbach, O. & Kutsch, W. (eds.) The nervous systems of invertebrates: An evolutionary and comparative approach, Basel: Birkhäuser, pp. 115-138.
39 Young, J. Z. (1963) “The number and sizes of nerve cells in octopus”, op. cit.
40 Shigeno, S.; Andrews, P. L. R.; Ponte, G. & Fiorito, G. (2018) “Cephalopod brains: An overview of current knowledge to facilitate comparison with vertebrates”, Frontiers in Physiology, 9 [accessed on 19 November 2020].
41 Hochner, B. (2010) “Functional and comparative assessments of the octopus learning and memory system”, Frontiers in Bioscience, 2, pp. 764-771.
42 Budelmann, B. U. (1995) “The cephalopod nervous system: What evolution has made of the molluscan design”, op. cit.