322 



COMPARATIVE 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 Lamellibranchiata, 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, and 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. Larvsc: j4 Of a Gastropod; B Later stage. C Of a PteropocT 

 (Cymbulia). v Velum, c Shell, js 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 LamelH- 

 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, G), 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 Grastropoda 

 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 



