428 



OPISTHOBRANCHIATA 



anterior 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 amount 

 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 result of the 



\-^P 



Fig. 286.— Illustrating the transition Fig. 287.— Illustrating the gradual covering 



of form in the shell of Tecti- of the shell in the Tectibranchiata by the 



hranchiata from the pointed spiral epipodia and mantle : A, Ilaminea ; B, 



to the almost flattened plate: A, Scaphander; C, Aplustrum ; D, Aplysia ; 



Adaeon ; "&, Ajjlustrum ; C, Cyli- E, Philine ; c.d, cephalic disc; ejo, ep, 



chna; D, Ati/s ; E, PhiUne ; T, epipodia; sA, shell. (Not drawn to scale.) 

 Dolahella ; G, A ply si a ; H, Pleu- 

 robranchus. (Not drawn to scale.) 



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 Ario7i (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 

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

 mud flats; others (Phi/Uirrhoe 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 



