MOLLUSC A. 



241 



The velum varies considerably in its development in different forms. In 

 the Hyalcid;u it is comparatively small and atrophies early; while in 

 Cymbulia (fig. 103) and the Gymnosomata it is large and bilobcd, and 

 persists till after the foot has attained 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, Nudibranchiata, etc. 



The shell-gland forms a pit at the aboral end of the body, and in 

 Cymbulia a chitinous 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 towards the anterior end of the body. On this everted 

 area a small plate is developed, which forms the commencement of the 

 embryonic shell with which the larvae of all Pteropods are provided. 



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 Hyaleidae the animal withdraws itself 

 from the larval shell, which becomes shut off from the permanent shell by a 

 diaphragm. The larval shell then becomes detached. 



In the Styliolidae the per- 

 manent shell becomes twice 

 the size of the embryonic 

 shell while the animal is still 

 in an embryonic condition, 

 but the larval shell persists 

 for life. In the Cymbulid.i 

 there is an embryonic and 

 secondary shell, which per- 

 sist together during larval 

 life. They are eventually 

 cast off at the same time 

 and replaced by a perma- 

 nent shell. 



In the Gymnosomata an 

 embryonic shell is develop- 

 ed, and a secondary shell 

 added to it during embryo- 

 nic life. Both are cast off 

 before the adult condition 

 is attained. After the shell 

 has been cast off three cili- 

 ated rings are developed (fig. 109). The anterior of these is placed between 

 the velum and the foot, and the two hinder ones on the elongated posterior 

 part of the body. 



B. II. lO 



B 



FIG. 109. FREE SWIMMING PNEUMODERMON 

 I.ARV.C. (After Gegenbaur, copied from Bronn.) 



The velum has atrophied in both larva:. 



In A three ciliated bands are present, and the 

 auditory vesicles are visible. 



In B the tentacles with suckers and the epi- 

 podia have become developed. 



an. anus. 



