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Comparative
As compared with the Struthiolariidae, the Aporrhaidae—their closest relatives—show very few advanced features in the nervous system. As described and figured by Bouvier, and personally checked by the writer in Aporrhais pes-pelicani, the cerebropleural and cerebropedal connectives are unusually long in this family, as are also the pleuropedals. The pedal ganglia lie much more posteriorly, having not yet attained the forward position they occupy in Struthiolaria. The subintestinal ganglion retains its primitive position, remote from the nerve ring, the right pleuroparietal connective being of equal length with the left and the right zygoneury correspondingly long.
The third family of the Superfamily Strombacea is the Strombidae, and in many respects—particularly in external form and mode of locomotion—these snails are much more highly specialised than either the struthiolariids or the aporrhaids. In the nervous system, however, they more nearly resemble the primitive family, the Aporrhaidae. By reference to earlier figures for Strombus gigas (see Simroth (1907), as checked by the writer's dissections of a species of Conomurex, it can be seen that the pedal ganglia retain their aporrhaid shape. The cerebropedal and pleuropedal commissures remain very long. The cerebral and pleural ganglia are in close contact, as in Struthiolaria, and in this feature a primitive character of the aporrhaids has been lost. The subintestinal ganglion is not incorporated in the nerve ring, but nevertheless shows a greater tendency to shift forward than in the Aporrhaidae, the right pleuroparietal connective being only about half as long as the left. In the visceral ganglia, all the Strombacea so far examined seem to be strikingly alike, and this resemblance extends to the details of the innervation of the gill, osphradium and left half of the mantle.
In general such trends as the approximation of ganglia—either the two members of a pair, or members of different pairs on the same side—and the shortening
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of commissures and connectives have been held to be reliable in indicating evolution from a “primitive” towards an “advanced” grade within a particular family or superfamily. In discussing the evolution of the Strombacea, the present writer (1950, 1951) has already considered that the family Aporrhaidae represents the primitive stock, and that the Strombidae and the Struthiolariidae are families advanced in various directions upon a basic aporrhaid pattern. The Struthiolariidae appear superficially the closer to the Aporrhaidae; they have avoided all of the more bizarre specialisations shown by the Strombidae, and may be said to have carefully observed the principle of “minimal adaptive specialisation”. Yet in many features of their mode of life, and in particular the acquisition of ciliary feeding, they have reached a higher level of organisation than either of the other two families. The present results on the nervous system have an added interest in giving emphasis to the distinctness of the struthiolariids from the aporrhaids or strombids, and in pointing clearly to the advanced position occupied by this family among the Strombacea.
As a measure of the relationships between the members of larger groups than superfamilies, the nervous system is likely to have a restricted value. For example, many of the more advanced families of prosobranchs have acquired a “concentrated” nerve ring by the incorporation of one or both of the parietal ganglia. In many cases this character must have been separately developed along parallel lines, and to try to incorporate in a single group all prosobranchs with a concentrated nerve ring is likely to bring about very misleading conclusions. Haller (see Simroth (1907)) attempted this in his recognition of “longicommissurate” and “brevicommissurate” Taenioglossa, with the result that the Aporrhaidae and the Strombidae are found in the first group, and their closest relatives, the Struthiolariidae, removed from them into the second.