MOLLUSCA. 229 



through the mouth and the shell-gland (s/isJ) : while the dorsal 

 surface is that opposite the ventral as already defined. 



Before the blastopore has attained its final condition three 

 organs make their appearance, which are eminently characteristic 

 of the typical molluscan larva. These organs are (i) the velum, 

 (2) the shell-gland, (3) the foot. 



The velum is a provisional larval organ, which has the form 

 of a praeoral ring of cilia, supported by a ridge of cells, often in 

 the form of a double row, the ventral end of which lies immedi- 

 ately 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. 27-i) 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 (Gegenbaur), the larva is stated to be coated by an 

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

 the researches of Fol have thrown very considerable doubt on 

 these statements. 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 pos- 

 terior and dorsal side. In this thickening a deep invagination 

 (fig. 101, s/is.} is soon formed, in which a chitinous plug may 

 become developed (Paludina, Cymbulia ? etc.), and in abnormal 

 larvae such a chitinous plug is generally formed. 



The foot is a simple prominence of epiblast on the ventral 

 surface, in the cavity of which there are usually a number of meso- 

 blast cells (fig. ioi,/). The larval form just described has been 

 named by Lankester the trochosphere larva. 



