I don’t have time in the foreseeable future to write up a review of how different invertebrates stack up against the three factors I’ve written about (although this is something I’d like to do at some point). Since I already had all the info to hand, I thought I’d jot down a very short summary anyway:
Octopodes (Cephalopoda)
Neuron count: Contain 500 million total neurons in the central nervous system with approx. 45 million in the central brain complex. This number of neurons is comparable with the number of neurons in some vertebrates.
Cortex/midbrain equivalent: Some studies have likened octopus brain functioning to that of mammalian cortex.
Centralisation: Possess a central brain region with important memory and learning functions. Unusually, the majority of neurons (about 300 million) are found in the semi-independent arms.
Insects (Arthropoda)
Neuron count: Typically have between 10^5 and 10^6 brain neurons.
Cortex/midbrain equivalent: Some evidence that the insect brain has functionality equivalent to that of the vertebrate midbrain. Debate exists as to whether a midbrain structure is sufficient to support consciousness.
Centralisation: Concentration of neurons into a clear brain structure which controls behaviour. Some ganglia distributed throughout the body.
Lobsters (Arthropoda)
Neuron count: Central nervous system contains around 10^5 neurons.
Cortex/midbrain equivalent: Crustacea (the subphylum which contains lobsters) are thought to have complex brains that may be comparable with those of insects. Therefore, there may be a midbrain-like structure similar to the potential one in insects. Debate exists as to whether a midbrain structure is sufficient to support consciousness.
Centralisation: Contain a central nervous system with a distinct brain region (the largest ganglion).
Snails (Gastropoda)
Neuron count: Typically have 10^3 to 10^4 brain neurons.
Cortex/midbrain equivalent: Possibly have a midbrain-like structure similar to the potential one in insects (I haven’t seen any research on this yet). Debate exists as to whether a midbrain structure is sufficient to support consciousness.
Centralisation: Have a functionally and structurally differentiated brain region which receives convergent sensory inputs and outputs motor commands. The ganglia distributed throughout the body contain a large proportion of the overall neurons.
Jellyfish (Cnidaria)
Neuron count: Typically contain somewhere in the region of 10^3 or 10^4 neurons.
Cortex/midbrain equivalent: Very unlikely to possess a structure analogous to a midbrain or cortex.
Centralisation: Nervous system mostly consists of nerve nets which perform de-centralised processing. A case can be made that sensory ganglia in jellyfish constitute a basic central nervous system where information integration occurs.
Clams (Bivalvia)
Neuron count: Contain somewhere in the region of 10^4 neurons.
Cortex/midbrain equivalent: Very unlikely to possess a brain structure similar to a midbrain or cortex.
Centralisation: Have a nervous system constituted of ganglia connected by nerve cords. No recognisable brain structure.
C. elegans (Nematoda)
Neuron count: 302 neurons in the hermaphrodite and 385 in the male.
Specific brain structure or equivalent: Very unlikely to possess a brain structure similar to a midbrain or cortex.
Centralisation: Contain a distinct brain structure.
I don’t have time in the foreseeable future to write up a review of how different invertebrates stack up against the three factors I’ve written about (although this is something I’d like to do at some point). Since I already had all the info to hand, I thought I’d jot down a very short summary anyway:
Octopodes (Cephalopoda)
Neuron count: Contain 500 million total neurons in the central nervous system with approx. 45 million in the central brain complex. This number of neurons is comparable with the number of neurons in some vertebrates.
Cortex/midbrain equivalent: Some studies have likened octopus brain functioning to that of mammalian cortex.
Centralisation: Possess a central brain region with important memory and learning functions. Unusually, the majority of neurons (about 300 million) are found in the semi-independent arms.
Insects (Arthropoda)
Neuron count: Typically have between 10^5 and 10^6 brain neurons.
Cortex/midbrain equivalent: Some evidence that the insect brain has functionality equivalent to that of the vertebrate midbrain. Debate exists as to whether a midbrain structure is sufficient to support consciousness.
Centralisation: Concentration of neurons into a clear brain structure which controls behaviour. Some ganglia distributed throughout the body.
Lobsters (Arthropoda)
Neuron count: Central nervous system contains around 10^5 neurons.
Cortex/midbrain equivalent: Crustacea (the subphylum which contains lobsters) are thought to have complex brains that may be comparable with those of insects. Therefore, there may be a midbrain-like structure similar to the potential one in insects. Debate exists as to whether a midbrain structure is sufficient to support consciousness.
Centralisation: Contain a central nervous system with a distinct brain region (the largest ganglion).
Snails (Gastropoda)
Neuron count: Typically have 10^3 to 10^4 brain neurons.
Cortex/midbrain equivalent: Possibly have a midbrain-like structure similar to the potential one in insects (I haven’t seen any research on this yet). Debate exists as to whether a midbrain structure is sufficient to support consciousness.
Centralisation: Have a functionally and structurally differentiated brain region which receives convergent sensory inputs and outputs motor commands. The ganglia distributed throughout the body contain a large proportion of the overall neurons.
Jellyfish (Cnidaria)
Neuron count: Typically contain somewhere in the region of 10^3 or 10^4 neurons.
Cortex/midbrain equivalent: Very unlikely to possess a structure analogous to a midbrain or cortex.
Centralisation: Nervous system mostly consists of nerve nets which perform de-centralised processing. A case can be made that sensory ganglia in jellyfish constitute a basic central nervous system where information integration occurs.
Clams (Bivalvia)
Neuron count: Contain somewhere in the region of 10^4 neurons.
Cortex/midbrain equivalent: Very unlikely to possess a brain structure similar to a midbrain or cortex.
Centralisation: Have a nervous system constituted of ganglia connected by nerve cords. No recognisable brain structure.
C. elegans (Nematoda)
Neuron count: 302 neurons in the hermaphrodite and 385 in the male.
Specific brain structure or equivalent: Very unlikely to possess a brain structure similar to a midbrain or cortex.
Centralisation: Contain a distinct brain structure.