MOLLUSC A. 



189 



figure. 



As a great protuberance on the 



FIG. 101. DIAGRAM OF AN EMBRYO OF PLEURO- 

 BRANCHIDIUM. (From Laukester.) 



/. foat; ot. otocyst; m. mouth; v. velum; 

 n fl. nerve ganglion; ry. residual yolk spheres; 

 s/w. shell-gland; i. intestine. 



in the position (i) in the 

 ventral surface is placed 

 the foot/. The long axis 

 of the body, at this period 

 though not necessarily in 

 the adult, is that passing 

 through the mouth and 

 the shell-gland (shs.) : 

 while the dorsal surface is 

 that opposite the ventral 

 as already defined. 



Before the blastopore 

 has attained its final con- 

 dition three organs make 

 their appearance, which 

 are eminently character- 

 istic of the typical mol- 

 luscan larva. These organs 

 are (1) the velum, (2) the 

 shell-gland, (3) the foot. 



The velum is a pro- 

 visional larval organ, which 



has the form of a pra3oral ring of cilia, supported by a ridge of 

 cells, often in the form of a double row, the ventral end of which 

 lies immediately dorsal to the mouth. Its typical position is shewn 

 in fig. 101, v. There are considerable variations in its mode and 

 extent of development etc., but there is no reason to think that it 

 is entirely absent in any group of Gasteropoda or Pteropoda. In a few 

 individual instances, especially amongst viviparous forms and land 

 Pulmonata, it has been stated to be absent. Semper (No. 274) failed 

 to find it in Vitrina, Bulimus citrinus, Vaginulus luzonicus, and 

 Paludina costata. It is very probably absent in Helix, etc. 



In some cases, e.g. Limax (Gegenbaur), Neritina (Claparede), 

 Pterotrachaea (Gegeubaur), the larva is stated to be coated by an uni- 

 form covering of cilia before the formation of the velum, but the 

 researches of Fol have thrown very considerable doubt on these state- 

 ments. In some cases amongst the Nudibranchiata (Haddon) and 

 Pteropoda there are one or two long cilia in the middle of the velar 

 area. In many Nudibranchiata (Haddon) there is present a more or 

 less complete post-oral ring of small cilia, which belongs to the velum. 



The cilia on the velum cause a rotation of the larva within the 

 egg capsule. Cilia are in most cases (Paludina, etc.) developed on 

 the foot and on a small anal area. 



The shell-gland arises as an epiblastic thickening on the posterior 

 and dorsal side. In this thickening a deep invagination (fig. 101, shs.) 

 is soon formed, in which a chitinous plug may become developed 

 (Paludina, Cymbulia ? etc.), and in abnormal larvee such a chitinous 

 plug is generally formed. 



