The anatomy and phylogeny of spondylus, with a particular reference to the Lamellibranch nervous system

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Abstract
The anatomy of Spondylus was investigated in order to ob-tain evidence of the phylogeny of the genus, with a view to a discussion of the evolution of the eyes of Pecten and Spondylus. Jackson, in his work on the phylogeny of the Pelecypoda, concluded that Spondylus had been evolved from Pecten. This theory was based entirely upon a study of the shell and upon paleontological discoveries[long dash]Pecten dates at least from Carboniferous times, Spondylus from the Triassic beds. The present work shows not only that the anatomical evidence is overwhelmingly in support of this view, but indicates the line which has been followed, and shows how the more equivalve species of Pecten, like P. opercularis, represent the earlier and more primitive type, and that the direction which culminated in Spondylus was that which can be seen in the Pectinidae, leading from P. opercularis through P. jacoboeus and P. maximus. .A surprising discovery is the presence of a thick nerve from the pedal ganglion to the visceral ganglion and the apparent absence of the cerebro-pedal connectives in Spondylus, a condition which appears to be unique in the Lamellibranchiata. A comparative study has been made of the nervous system of Spondylus and Pecten with a view to the explanation of these extraordinary divergencies from the ordinary type. This study indicates that Spondylus must be regarded as the extreme of an evolutionary line, which probably commenced with the aviculopectens of Palaeozoic times.

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