428 



OPISTHOBRANCHIATA 



CHAP. 



portion of the body, and do not become reflected, they may, as 

 in most Pteropoda Thecosomata, not directly affect the shell. 

 But when, as in the Tectibranchiata, the epipodia are medio- 

 lateral, and tend to envelope the shell, their effect may be 

 traced by a series of forms varying in proportion to the amonnt 

 of shell-surface covered by the epipodia. The two principal 

 lines along which modification takes place are the gradual 

 reduction of the spiral nature of the shell, and the gradual 

 lessening of its solidity. Both these changes are the direct 



Fig. 286. — Illustrating the transition 

 of form in the shell of Tecti- 

 branchiata from the pointed spiral 

 to the almost flattened plate: A, 

 Actaeon; B, Aplustrum ; C, Cyli- 

 chna ,• D, Atys ; E, PhiUne ; F, 

 Dolahella : G, Aplysia ; H, Pleu- 

 robranchus. (Not drawn to scale.) 



Fig. 287. — Illustrating the gradual covering 

 of the shell in the Tectibranchiata by the 

 epipodia and mantle : A, Haminea ; B, 

 Scaphander ; C, Aplustruin ; D, Aplysia ; 

 E, Philine ; c.d, cephalic disc ; ep, ep, 

 epipodia ; sh , shell. (Not drawn to scale.) 



result of the additional protection afforded to the visceral mass 

 by the reflected epipodia, which renders the existence of a shell 

 less and less necessary. A precisely similar line of change is 

 seen in the Pulmonata, culminating in forms like Avion (p. 174). 

 The habits of life of the Opisthobranchiata are very varied. 

 Some, especially the heavier types, burrow in sand, and are then 

 usually furnished with a broad cephalic disc, as a digging appa- 

 ratus ; some (certain Bulla) flit about in shallow pools on mud 

 flats; others (^Phyllirrhoe and the Pteropoda) swim freely in 

 the open sea ; others (most Nudibranchiata) crawl slug-like on 

 sea-weeds or corallines, and in colour singularly harmonise with 



