MOLL USC A. 



199 



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 Gasteropods. There are usually only three hypoblastic 

 spheres at the close of the segmentation in the Thecosomata, and a some- 

 what larger number in the Gymnosomata. The blastopore 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 (tig. 109). In most forms of Pteropods the 

 dorsal part of the body, covered by the mantle, is produced into a visceral 

 sack like that of the Cephalopoda (fig. 108). 



The velum varies considerably in 

 its development in different forms. 

 In the Hyaleidpe it is comparatively 

 small and atrophies early ; while in 

 Cymbulia (fig. 103) and the Gymno- 

 somata it is large and bilobed, and 

 persists till after the foot has at- 

 tained its full development. 



The free edge of the velum is 

 provided with long motor cilia, and 

 its lower border with small cilia which 

 bring the food to the mouth. In 

 Cleodora there is a median bunch of 

 cilia in the centre of the velum like 

 that in the Lamellibranchiata, Nudi- 

 branchiata, etc. 



The shell-gland forms a pit at the 

 aboral end of the body, and in Cym- 

 bulia a chitiiious plug appears to be 

 normally formed in this pit. The 

 pit afterwards everts itself. The 

 edge of the everted area becomes 

 thickened and gradually travels to- 

 wards the anterior end of the body. 

 On this everted area a small plate is 

 developed, which forms the com- 

 mencement of the embryonic shell 

 with which the larvae of all Pteropods 

 are provided. 



FIG. 108. EMBRYO OF CAVOLINIA 

 (HYALEA) TRIDENTATA. (After Fol). 



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

 intestine; jr. nutritive sack; mb. mantle ; 

 me. mantle cavity ; Kn. contractile sinus; 

 h. heart; ?. renal sack; /. foot; pn. epi- 

 podia ; q. shell ; ot. otolithic sack. 



The remainder of the embryonic 

 shell is secreted in successive rings 

 by the thickened edge of the mantle, and grows with this till it reaches the 

 neck (fig. 108). The permanent shell is added subsequently, usually on a 

 very different model to the larval shell. The fate of the embryonic shell is 

 very various in different forms. In the Hyaleidre tho animal withdraws 



