APLYSIA. 195 



upon frilled Chondrus crispus possessed frilled parapodia 

 like the fronds of the weed. The resemblance was so perfect 

 that unless the Aplysia was moving, it was impossible to 

 distinguish it from its surroundings. 



The greyish flecks on the skin differ in shape and size at 

 different ages and in different specimens. When first they 

 appear they are spots, few and far between. After a time 

 ovate flecks arise, arranged radially round the original spot, 

 like the zooids of a compound Ascidian. The radial spots 

 finally fuse with one another and with the first spot to form 

 an irregular patch. These spots bear a close and unmistakable 

 resemblance to the irregular colonies of Polyzoa found growing 

 on the weed. 



The body is soft and slimy to the touch owing to the 

 mucus which is secreted abundantly by the epithelial glands 

 of the skin. 



Externally the animal appears bilaterally symmetrical, 

 but the symmetry is superficial and does not extend to the 

 internal organs. It has been acquired by detorsion of a spirally 

 coiled type. The body may be divided, for purposes of 

 description, into three regions : — 



(1) Cephalic region. 



(2) Foot and its appendages. 



(3) Mantle and Visceral Hump. 



1. Cephalic region. The anterior portion of the body, 

 consisting of the head and neck, is very highly contractile. 

 This region is frequently bent ventralwards when Aplysia is 

 moving sluggishly over the weeds. (Fig. 2.) The head bears 

 the mouth, two pairs of auriculate tentacles, a pair of sessile 

 eyes and the male copulatory organ. The tentacles are 

 contractile, but not, as in the snail or slug, invaginable. The 

 anterior pair are short, with an open groove on the outer side 

 formed by the curling of the edge. At the base they are 

 stout, and confluent with paired labial expansions which form 



