198 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



along the edges of the foot, and the type is called 

 retrograde. 



Aplysia can also swim by means of sinuous movements 

 of its parapodia, or can, according to Guiart, jerk itself suddenly 

 backwards like a Cephalopod by rapid closing of the parapodial 

 lobes. This last movement is probably connected with respira- 

 tion and with the excretion of faecal matter. 



3. Mantle and Visceral Hump. (See Fig. 7.) 



[Hold the parapodia apart with the fingers.] 



Between the lobes of the upstanding parapodia and overlying 

 the visceral hump can be seen a mushroom-shaped mass 

 consisting of the mantle enclosing the shell. In the typical 

 Gastropod the mantle encloses the projecting visceral mass, 

 and mantle and visceral mass are permanently lodged within 

 a conical or spirally coiled shell (Cf. Patella, Buccinum, etc.). 

 Between the mantle and the body is a space, the pallial cavity, 

 which contains the ctenidia, and into which the anus, the 

 kidney, and the genital glands discharge. At first sight the 

 condition in Aplysia appears to vary from this type, but is 

 really a modification of it, due to reduction of the mantle and 

 shell and to the reflection of the mantle flaps over the shell. 

 In many Gastropods the mantle edges become either tem- 

 porarily or permanently reflected dorsalwards over the surface 

 of the shell (e.g., Cypraea, Physa, etc.). In Aplysia this has 

 become permanent by the fusion of the mantle flaps in the 

 mid-dorsal line. The fusion, however, is not quite complete, 

 as a circular aperture (Fig. 7, sh. ap.), which varies in size in the 

 different species of Aplysia, is left in the centre. In the young 

 individual the space is very much larger than in the adult. 

 (Cf. Fig. 21). Through the mantle aperture can be seen the 

 delicate, transparent, horny shell. As might be expected, 

 the enclosure of the shell is causing its gradual disappearance. 

 In some allied forms it has disappeared altogether. The six 



