The Origin of Vertebrates

by Walter Holbrook Gaskell

If the argument used so far is correct, and this tripartite system of nerve-roots, as seen in the cranial nerves of the vertebrate, really represents the original scheme of innervation in the palæostracan ancestor, then it follows that each segment of Limulus ought to be supplied by three nerves—(1), a sensory nerve supplying its own portion of the skin-surface of the prosomatic and mesosomatic carapaces; (2), a lateral mixed nerve supplying exclusively the appendage of the segment, for the appendages carry the respiratory organs; and (3), a motor nerve supplying the body-muscles of the segment.

It is a striking fact that Milne-Edwards describes the nerve-roots in exactly this manner. The great characteristic of the nerve-roots in Limulus as in other arthropods is the large appendage-nerve, which is always a mixed nerve; in addition, there is a system of sensory nerves to the prosomatic and mesosomatic carapaces, called by him the epimeral nerves, which are purely sensory, and a third set of roots which are motor to the body-muscles, and possibly also sensory to the ventral surface between the appendages.

Moreover, just as in the vertebrate central nervous system the centres of origin of the motor nerves of the branchial segmentation are distinct from those of the somatic segmentation, so we find, from the researches of Hardy, that a similar well-marked separation exists between the centres of origin of the motor nerves of the appendages and those of the somatic muscles in the central nervous system of Branchipus and Astacus.

In the first place, he points out that the nervous system of Branchipus is of a very primitive arthropod type; that it is, in fact, as good an example of an ancient type as we are likely to find in the present day; a matter of some importance in connection with my argument, since the arthropod ancestor of the vertebrate, such as I am deducing from the study of Ammocœtes, must undoubtedly have been of an ancient type, more nearly connected with the strange forms of the trilobite era than with the crabs and spiders of the present day.

His conclusions with respect to Branchipus may be tabulated as follows:—

1. Each ganglion of the ventral chain is formed mainly for the innervation of the appendages.

2. Each ganglion is divided into an anterior and posterior division, which are connected respectively with the motor and sensory nerves of the appendages.

3. The motor nerves of the appendages arise as well-defined axis-cylinder processes of nerve-cells, which are arranged in well-defined groups in the anterior division of the ganglion.

4. A separate innervation exists for the muscles and sensory surfaces of the trunk. The trunk-muscles consist of long bundles, from which slips pass off to the skin in each segment; they are thus imperfectly segmented. In accordance with this, a diffuse system of nerve-fibres passes to them from certain cells on the dorsal surface of each lateral half of the ganglion. These cell-groups are therefore very distinct from those which give origin to the motor appendage-nerves, and, moreover, are not confined to the ganglion, but extend for some distance into the interganglionic region of the nerve-cords which connect together the ganglia of the ventral chain.

Hardy's observations, therefore, combined with those of Milne-Edwards, lead to the conclusion that in such a primitive arthropod type as my theory postulates, each segment was supplied with separate sensory and motor somatic nerves, and with a pair of nerves of mixed function, devoted entirely to the innervation of the pair of appendages; that also, in the central nervous system, the motor nerve-centres were arranged in accordance with a double set of segmented muscles in two separate groups of nerve-cells. These nerve-cells in the one case were aggregated into well-defined groups, which formed the centres for the motor nerves of the markedly segmented muscles of the appendages, and in the other case formed a system of more diffused cells, less markedly aggregated into distinct groups, which formed the centres for the imperfectly segmented somatic muscles.