240 



HETEROPODA. 



stomach. The remainder, which unites with the proctodsum, forms the 

 intestine. 



The structural peculiarities of the adult are formed by a post-larval 

 metamorphosis. The caudal appendage of Pterotrachea and Firoloidea is 

 formed as an outgrowth of the upper border of the hind end of the foot. 

 The so-called fin arises as a cylindrical process in front of the base of the 

 foot, which is eventually flattened laterally. In the Atlantidae it is in some 

 cases at first vermiform, and in other cases attains directly its adult struc- 

 ture. The embryonic foot itself gives rise in Pterotrachea, Firoloidea and 

 Carinaria to the tail, on the dorsal and posterior side of which the operculum 

 may still be seen in young specimens. In Atlanta it forms the posterior part 

 of the foot on which the operculum persists through life. 



The embryonic shell is completely lost in Pterotrachea and Firoloidea, 

 and the shell is rudimentary in Carinaria. With its atrophy the mantle 

 region also becomes much reduced. 



The velum is enormously developed in many Heteropods. In Atlanta it 

 is six-lobed, each of the two primitive lateral lobes being prolonged into 

 three processes, two in front, and one behind. As in all other cases, it 

 atrophies in the course of the post-larval metamorphosis. 



Pteropoda. The early larval 

 form of the Pteropods is closely 

 similar to that of marine Gastero- 

 pods. There are usually only three 

 hypoblastic spheres at the close of 

 the segmentation in the Thecoso- 

 mata, and a somewhat larger num- 

 ber in the Gymnosomata. The blas- 

 topore closes at the oral region, on 

 the nutritive side of the ovum, and 

 the shell-gland is placed at the 

 original formative pole. The velum, 

 shell-gland and foot have the usual 

 relations. Although many of the 

 adult forms are symmetrical, there is 

 very early an asymmetry visible in 

 the larva, shewing that the Pteropods 

 are descended from asymmetrical 

 ancestors. In the Gymnosomata 

 there is a second larval stage after 

 the loss of the shell when the larva 

 is provided with three rings of cilia 

 (fig. 109). In most forms of Ptero- 

 pods the dorsal part of the body, 

 covered by the mantle, is produced 

 into a visceral sack like that of the 

 Cephalopoda (fig. 108). 



FIG. 108. EMBRYO OF CAVOLINIA 

 (HYALEA) TRIDENTATA. (After Fol.) 



m. mouth ; a. anus ; s. stomach ; i. 

 intestine ; <r. nutritive sack ; nib. mantle ; 

 me. mantle cavity ; Kn. contractile sinus ; 

 h. heart ; r. renal sack : f. foot ; pn. epi- 

 podia ; q. shell ; ot. otolithic sack. 



