TYPE MOLLUSCA. 321 



leads a free-swimming pelagic existence, assuming the adult 

 habit only after a further growth which is accompanied by a 

 reduction of the velum. 



Such a Veliger larva occurs in the life-history of the majority of the 

 Gasteropoda, though, as might be expected, it undergoes certain modifica- 

 tions more especially in terrestrial forms, though even in these there are 

 ample indications of its existence. Indeed the Veliger is so frequent in its 

 occurrence that the conclusion is almost unavoidable that it has an ances- 

 tral significance and represents in a more or less modified condition a 

 primitive form from which the MoUusca have descended. A comparison of 

 the Veliger with the Annelid Trochophore brings out, as already men- 

 tioned, numerous similarities. These are especially noticeable in the ar- 

 rangement of the ciliary bands, which resemble those of the Trochophore 

 part for part, even to the dorsal break in their continuity. It is difficult 

 to believe that such marked similarities should have been acquired inde- 

 pendently in the larvae of two different groups of animals and have become 

 so characteristic, a difficulty rendered all the greater by the occurrence of 

 other points of similarity, such as the development of the mesoderm, in 

 some forms at least, from a pair of mesoblasts situated at the posterior 

 extremity of the blastoccel ; the existence of a thickening of the ectoderm 

 in the centre of the velar area in some forms, corresponding to the apical 

 plate of the Trocophore ; and the occurrence of a larval excretory organ or 

 nephridium in some Veligers which may be compared to the larval ne- 

 phridium or head-kidney of the Trochophore. The probable significance of 

 this larval form will be more suitably discussed at the conclusion of this 

 chapter ; it remains to be said here regarding it that the occurrence among 

 the Pteropods of larvae with several bands of cilia surrounding the visceral 

 hump is probably to be explained as a secondary adaptation, just as the 

 mesotrochal Annelid larvae are probably secondary modifications of a Tro- 

 chophore. 



As regards the relationships of the various groups of Gasteropods 

 among themselves, there is little doubt but that the Diotocardiate Proso- 

 branchs are, on the whole, the most primitive of all the groups and stand 

 nearest to the Amphineura, and from them the Monotocardia have devel- 

 oped. The Opisthobranchs and Pulmonates are apparently closely re- 

 lated, the latter group having been derived from Tectibranchiate ancestors 

 somewhat more generalized probably than any Opisthobranch now living. 

 The orthoneurous character of the nervous system and the structure of the 

 reproductive system in the two groups indicates their affinity, and it seems 

 probable that the Pulmonates are to be regarded as Opisthobranchs which 

 have accommodated themselves at first to an amphibious life, somewhat 

 similar to that now led by Onchidium, and later to one purely terrestrial, 

 at the same time differentiating an organ for aerial respiration. Such an 

 origin would imply that the aquatic species have secondarily taken to fresh 

 water as a habitat, having originally been terrestrial, an idea which on 



