332 COMPARATIVE ANATOMY. 



along the median line by means of the u'ncalcified portion of the 

 shell. The "hinge" is formed at the point where the two valves 

 pass into one another ; the non- calcified chitinons substance which 

 connects the two shells forms the ligament. Its layers pass into 

 those of the shells ; and the two valves are seen to be merely parts 

 of a structure which is rudimentarily single, and later on becomes 

 so again, and is homologous with the shells of the rest of the 

 Mollusca. Near the ligament the valves form alternating and inter- 

 locking processes (cardinal teeth) which serve to close the shell more 

 perfectly. 



The shells of the Gastropoda are most markedly distinguished 

 from those of the Lamellibranchiata by the absence of any uncalcified 

 portion. They are not unfrequently internal. 



This internal position is generally found in those Tectibranchiata 

 that have a rudimentary shell, and in some Pulmonata. In the latter 

 (Helicinaa) the shell very soon becomes external, while in others 

 (Limacina?) it remains rudimentary, and placed within the mantle. 

 Sometimes it merely consists of a few calcareous concretions. 



The various stages of atrophy are also to be seen in the shells of 

 some other divisions, as, for example, in the Heteropoda, in which 

 the rudimentary shell of Carinaria is intermediate between the shell 

 of Atlanta, which covers in the whole body, and that of the Ptero- 

 trachea?, where it is altogether absent. But these latter have a tem- 

 porary shell during their larval life, which covers in the whole body, 

 just as it does in the Opisthobranchiata, which are also shell-less in 

 later life. Its general presence points to its being a common 

 heritage of all the Gastropoda, some divisions of which lose it early. 

 The Thecosomatous Pteropoda resemble the Gastropoda in forming a 

 shell. 



The animal does not always occupy the whole of the shell. Many 

 Gastropoda withdraw themselves from the end of their shell as 

 growth proceeds ; and the end is then shut off by a layer of shell- 

 substance. The same thing happens in some Pteropoda (Chresei's), 

 and indicates the commencement of that arrangement which is so 

 much more distinctly marked in the Cephalopoda. 



The substance of the shell, which is a product of excretion from 

 the mantle, varies very greatly, from the soft structures of some to the 

 firm solid parts which form the shells of most Prosobranchiata. The 

 soft form of shell consists of organic substance merely. Shells 

 become firmer and horny in character when impregnated with cal- 

 careous salts ; and when the inorganic substance forms the greater 

 part of the shell we get strong coverings. 



The simple condition of the cup-shaped embryonic shells persists 

 in some Gastropoda, and by growing regularly gets to have a more or 

 less flattened or conical shape (e.g. Patella) ; in most, however, it 

 becomes spiral by growing out unequally, and these spiral forms may 

 again undergo all kinds of modifications. As the embryonic shells 

 serve to shelter the whole body, even in those which lose them later 

 on, we must look for the typical form in them ; from it all the other 



