188 GASTROPODA. 



unequal growth which leads to the coiling of the shell, has already 

 beeu described (p. 147) and so have the special shapes assumed by 

 the shell (e.g., Pteropoda) and the partial or total loss of the shell in 

 the Heteropoda, Opisthobranchia, Pteropoda and Pulmoriata. 



It is a striking fact that a few specially low forms of Gastropoda 

 such as Haliotis and still more Patella, and Fissure!! a, are dis- 

 tinguished by a reduction of the coils and the adoption of a flat cup- 

 shaped shell. In youth, the shell was, as in other Gastropods, 

 distinctly coiled. This can be seen particularly well in Fissiirella 

 (Fig. 85 A-C). The margin of the shell is at first unbroken, but a 

 slit appears in it later lying above the slit which occurs in the 

 mantle of these forms (Fig. 85 4). The shell-slit is of special interest 

 because it is present in two of the oldest fossil Gastropods, e.g., 

 Pleurotomaria and Bellerophon, both of which are found in the 

 Cambrian.* The ontogeny of Fissurella would suggest that these 

 forms with slit shells have been derived from forms in which the 

 margin is not slit. In many forms the slit is retained as such 

 (Scissurella, Emarginu/a, and fossil as well as recent Pleurotomariae), 

 in others, as the shell grows further ; the most posterior portion of 

 the slit becomes cut off by shelly matter from the rest of the slit and, 

 as this continues to take place throughout life, we find in such forms 

 as Haliotis a series of consecutive apertures in the shell ; in other 

 cases, the slit becomes to a great extent closed by a shell-substance 

 of peculiar structure which is seen extending along the length of the 

 whorls as the slit-band. In Fissurella, the margin, as it grows 

 further, is unbroken (Fig. 85 B). The reduction of the coiled part 

 of the shell and the fairly equal growth of the whole margin leads 

 finally to the slit taking up a central position near the apex of the 

 adult shell (Fig. 85 6'). The shell of Fissurella has now passed from 

 a coiled form to that of a flattened cone ; this change is due, as in 

 other Gastropods with similarly simple shells, to the manner of life 

 and, as ontogeny shows, must be regarded as a phenomenon of 

 degeneration. The symmetry of the shell is thus of a secondary 

 character. 



B. The Nervous System. 



The nervous system usually arises by delamination (Fig. 88, eg, 

 pi, p, p. 194), but it cannot be doubted that, according to recent 



* A description of the development of the Gastropoda at the different 

 geological epochs has been given by KOKEN (No. 56). See also ZITTEL'S 

 Palaeontologie. 



