APLYSIA. 197 



extending forwards in front of the buccal veil and backwards 

 as a projection or k ' tail " posterior to the visceral mass. Its 

 anterior end is almost rectangular, the posterior pointed. There 

 is no division into regions as in many Gastropods. Anteriorly 

 the foot is separated from the body by a backwardly directed 

 gutter (Figs. 2 and 22, gt.), which makes a cleft about half an 

 inch deep between the foot and the buccal veil. The vertical 

 slit-like mouth is continuous ventrally with this gutter, but 

 behind the mouth the gutter narrows and ends blindly. (Vide 

 glands of the foot.) 



From the postero-lateral portion of the foot, in the region 

 of the visceral hump, arise a pair of upwardly directed out- 

 growths, the parapodia or swimming lobes. These are mobile 

 fleshy structures and cover the visceral hump when the animal 

 is at rest. Anteriorly the flaps are free, their bases being 

 about an inch apart, but posteriorly they are fused completely 

 in this species. The line of attachment is thus a horseshoe, 

 with the open ends directed forwards. (Fig. 4, para. ; Fig. 7, 

 para, b.) 



The foot is much narrower than in the land snail, and the 

 whole of it is rarely used for creeping. Usually one portion 

 of it is flattened and the remainder has the edges curled 

 towards one another. When Aplysia crawls up the stalks 

 of seaweeds only the anterior part of the foot is flat, the posterior 

 portion curls itself round the stalk by approximation of the 

 lateral edges. (Figs. 21-23.) The narrowness and elongation of 

 the foot are well adapted for this purpose. (Compare the broad 

 flat foot of a Doris.) It is stated by various observers that the 

 foot can also be used for crawling in the inverted position on 

 the surface film as in many Nudibranchs. During creeping 

 and swimming the foot exhibits waves of movement due to 

 muscular contraction. These are not so well shown in Aplysia 

 as in the Snail, and different observers give conflicting accounts 

 of the direction of the waves. They appear to pass backwards 



