304 



INVEETEBEATA 



CHAP. 



OTHER GASTROPODA 



It has been mentioned above that the metamorphosis into the 

 adult form, so far as its external features are concerned, has been 

 worked out in Acmaea, Fissurella, and Halwtis by Boutan (1899). 

 Taking Acmaea as an example, since it is closely allied to Patella, 

 we find, according to Boutan, that as the visceral hump becomes 

 longer, the alimentary canal, consisting of larval stomach and 

 intestine, becomes bent into a U shape ; the mouth and anus being 

 comparatively near each other, separated only by the small foot. 

 Suddenly the incipient mantle-cavity, and the anus which opens into 

 it, which were originally situated in the middle line behind, become 

 twisted upwards and forwards so as to open on the neck of the 



ten 



muse. 



en 



int 



h 



Fio. 240. Two views of the advanced stages of development of Acmaea rinjinea. 



(After Boutan.) 



A, Lateral view of veliger larva. B, Dorsal view of youu.n .{rumen just after the velum has brni cast 

 off. a, anus ; /, foot ; gl, glands in the mantle roof ; int, intestine ; muse, retractor muscle of shell ; 

 oc, eye ; up, operculum; ot, otocyst ; pst, peristome ; sh, larval shell ; ten, tentacle ; V, velum. 



larva. According to Boutan this torsion, which is due to the 

 unequal growth of the two sides of the larva, takes place with 

 great rapidity, and as a result of it the apex of the visceral hump 

 hangs backwards (cf. Figs. 237 and 238). 



The growth of the mantle edge continues to be uniform all round 

 its periphery, and produces an everted, conical lip to the shell, which 

 is termed the peristome. At the sides of the apical plate the eyes 

 and tentacles have made their appearance. Soon the velum is 

 cast off and the young mollusc sinks to the bottom. The peristome 

 continues to grow until it forms the adult conical shell ; finally the 

 visceral hump is withdrawn from the tiny cap-like larval shell and it 

 is cast off, and thus the adult state is attained. There is no reason- 

 able doubt that the later development of Patella is in all respects 

 similar to that of Acmaea. 



