

APLYSIA. 199 



genera of Aplysiidae can be arranged in a series on this point, 

 thus : — 



Dolabella. Shell with deep anal incision and short 

 spire. 



Dolabrifer. Boat-shaped shell. 



Aplysiella. Shell slightly covered. 



Aplysia. Shell covered, circular aperture in mantle. 



Notarchus. Shell much reduced, shell sac closed. 



Phyllaplysia. Shell absent. 

 As the result of the inclusion of the shell by the mantle 

 and of the compression of the visceral hump during the 

 acquisition of secondary symmetry, the mantle cavity, or cavity 

 between the mantle and the body wall, is much reduced. 



The mantle, shell, and visceral hump form a projecting 

 shelf, fixed on the left side but free on the right side of the 

 animal. The mantle is uniformly thin dorsally, but at the right 

 postero-lateral corner it becomes thick and fleshy and curves 

 upwards in the form of a spout or siphon known as the anal 

 spout or funnel. This is shaped somewhat like the auriculate 

 tentacle as it is open along its outer side, but it is much thicker. 

 The edges are often frilled. On the posterior wall of this 

 spout lies the anus, which has a radiate appearance due to the 

 sphincter muscle which regulates the opening and closing of 

 the aperture. When the parapodia are closed the anal spout 

 frequently projects between them, but it can be completely 

 withdrawn. (Cf. Figs. 4, 5, an. f.) 



[Lift the free edge of the mantle shelf.] 



Beneath the mantle shelf lies a crescent-shaped space, 

 open dorsally and anteriorly where the parapodia are free, 

 but shut in posteriorly where they are fused. The concave 

 side of this crescent and its floor are formed by the body wall, 

 its outer or convex side is bounded by the right parapodium, 

 and the incomplete roof consists of a glandular projection of 



N 



