322 



COMPAKATIVE ANATOMY. 



The size of this velum is correlated with the differentiation of 

 the head, from the upper surface of which it is developed ; it is in 

 some Pteropoda only that the head, once formed, undergoes any 

 considerable atrophy. 



Just as in the Lainellibranchiata, the mantle rises up in the 

 form of a fold of the body- wall, which covers over the dorsal surface 

 and forms the shell on its outer side. As this dorsal area of the 

 body — which is surrounded by the mantle-fold, and the shell, 

 which is being developed into its house — continues to bulge out, it 

 gradually forms a blind sac, which soon contains the greater part of 

 the viscera (visceral sac) ; in this way the viscera come under the 

 direct protection of the shell. As development goes on, the mantle- 

 fold becomes less intimately connected with the body, and gives 

 rise, inferiorly, to a wider cavity, in which the growing gills are 

 contained, aud which is homologous with the branchial cavity of the 

 Lamellibranchiata (Fig. 167, A B). This development of a fold of 

 the integument into the mantle, and the consequent appearance of a 



Fig. 170. Larvse: A Of a Gastropod; B Later stage. C Of a Pteropod 

 (Cymbulia). v Velum, c Shell, p Foot, op Operculum, t Tentacles. 



subjacent space, the branchial cavity — which looks like an invagi- 

 nation from the exterior — undergoes modifications, which are largely 

 due to the formation of the shell. In consequence of the mantle 

 growing unequally on either side, and not equally, as in the Lamelli- 

 branchiata, and from the fact that it is principally developed at one 

 point in connection with the development of the shell, the branchial 

 cavity comes to be a single cavity, placed in the same region. This 

 region is either beneath the hinder portion of the mantle, as in the 

 Pteropoda (Fig. 1 70, C), or beneath the anterior portion, as in most 

 of the Gastropoda (B). The want of symmetry, which is due to the 

 coiling of the shell, causes the branchial cavity of most Gastropoda 

 to lie on one side ; this is an adaptation to the larger amount of 

 space which is afforded by the lateral portion of the shell. The 

 production of the unilateral and asymmetrical branchial cavity from 

 a paired and symmetrical space is proved by numerous facts ; so 

 that we are led to think that the asymmetry of the shell is probably 

 a secondary arrangement. 



A number of degenerate and more perfect arrangements have 



