MOLLUSC A. 



191 



The edge of the mantle forms a projecting lip separating the 



FIG. 103. LABVJE OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE. 



(From Gegenbaur.) 



A, and B. Earlier and later stage of Gasteropod. C. Pteropod (Cynibulia). 

 v. velum; c. shell; p. foot; op. operculurn; t. tentacle. 



dorsal visceral sack from the head and foot. An invagination 

 appears, usually on the right in Gasteropods, and eventually extends 

 to the dorsal side (fig. 103 B). It gives rise to the pallial or 

 branchial cavity, and receives also the openings of the digestive, 

 generative and urinary organs. In most Pteropods it is also formed 

 to the right, and usually eventually extends afterwards towards the 

 ventral surface (fig. 103 G). In the pallial cavity the gills are 

 formed, in those groups in which they are present, as solid processes 

 frequently ciliated. They are coated by epiblast and contain a core 

 of mesoblast. They soon become hollow and contractile. 



The velum in the more typical forms loses its simple circular 

 form, and becomes a projecting bilobed organ, which serves the 

 larva after it is hatched as the organ of locomotion (fig. 103 B 

 and C). The extent of the development of the velum varies greatly. 

 In the Heteropods especially it becomes very large, and in Atlanta 

 it becomes six-lobed, each lateral half presenting three subdivisions. 

 It is usually armed on its projecting edge with several rows of long 

 cilia, and below this with short cilia which bring food to the mouth. 

 It persists in many forms for a very long period. Within the area 

 of the velum there appear the tentacles and eyes (fig. 103 B). 

 The latter are usually formed at the base of the tentacles. 



The foot grows in most forms to a very considerable size. On 

 its hinder and dorsal surface is formed the operculum as a chitinous 

 plate which originates in a depression lined by thickened epiblast, 

 much in the same way as the shell (fig. 103 B and C, op). In 

 the typical larval forms it is only possible to distinguish the anterior 

 flattened surface of the foot for locomotion and the posterior opercular 

 region, but special modifications of the foot are found in the Pteropods 

 and Heteropods, which are described with those groups. The foot 

 very often becomes richly ciliated, and otic vesicles are early de- 

 veloped in it (fig. 101, of). 



