vii MOLLUSGA ONTOGENY 257 



perhaps usually, remains open ; notwithstanding this, the oesophagus arises by the 

 sinking in of ectoderm cells. 



Paludina is, as far as is known, the only Mollusc in which the mesoderm 

 originates as an outgrowth of the archenteron. This fact is no doubt connected 

 with its poverty in nutritive yolk. In other Gastropods, the mesoderm arises in the 

 manner already described for other Molluscs, as two large symmetrical primitive 

 cells, at the posterior edge of the blastopore ; these cells look more like endodermal 

 than ectodermal cells, and soon pass into the segmentation cavity. 



A Veliger larva, i.e. a Trochophora with Molluscan characteristics, always forms 

 (1) a dorsal shell gland with the embryonic shell, 

 and (2) a ventral rudiment of the foot. 



The outward appearance of the Yeliger larva, 

 however, varies much in different groups, the 

 variations being connected with the manner of 

 life and of feeding of the embryo. 



In the marine Gastropods, i.e. in the majority 

 of the Prosobranchia (including the Hetcropoda). 

 the Pulmonate genus Oncidium, and all Opistho- 

 branchia, the embryo leaves the egg envelope early, 

 as a free-swimming Yeliger larva. In all these 

 forms, the preoral ciliated ring is well developed. 

 The ectodermal floor of the ciliated ring usually FIG. 213. Larva of Cymbulia 

 bulges out anteriorly, so that the cilia appear to (Pteropod), from the left side (after 

 be carried by a distinct circular ridge. This ridge Gegenbaur) i, velum ; 2, shell ; 3, 

 J . parapodia (fins); 4, foot with oper- 



even grows out laterally to form a lobe of varying ^ lu {^ 5) 



size, which carries at its edge long and strong 



cilia, and is occasionally itself produced into an upper and a lower 'lobe. This is 

 the true velum of the free-swimming Gastropod larva, and is its only organ of 

 locomotion. It is internally traversed from wall to wall by contractile mesoderm 

 cells (muscle cells), which make it highly contractile. In the older larvae, the head 

 with the velum can be withdrawn into the shell. 



It is probable that the velum of the larva also serves for respiration, and perhaps 

 for bringing about a circulation of the body fluid by means of its contractility. 



The embryos of fresh-water and terrestrial Gastropods, where these animals are 

 not viviparous, remain longer in the egg, and leave it only after their transformation 

 into young Gastropods, the larval organs (the velum, the primitive kidney, the 

 cephalic vesicle, and the pedal vesicle or podocyst) having degenerated within the 

 egg envelope. Even in these forms, the mass of nutritive yolk contained in the egg 

 is not very great, but there is a large quantity of albumen stored up within the egg 

 capsule, which serves as food for the developing embryo ; this is either absorbed 

 through the body wall or swallowed. . The egg capsules are always large, in some 

 cases (in tropical terrestrial Gastropods) as large as the egg of a small bird ; but 

 their size is not, as in the Cephalopoda, determined by that of the egg contained, 

 but by the quantity of albumen in which the small egg is embedded. The mature 

 egg capsule contains a young Gastropod of considerable size with a well-developed 

 shell. 



In terrestrial and fresh-water forms, the velum is not needed as a locomotory 

 organ, and is therefore reduced to a single ring of cilia or to two lateral ciliated 

 streaks. It is entirely wanting in the embryos of a few terrestrial Gastropod snails. 

 The respiratory and circulatory functions, which were originally merely accessory 

 functions of the velum, here become of greater importance. The nuchal region 

 becomes much bulged forward, and forms a cephalic vesicle (Fig. 214), which is 

 sometimes very large, and undergoes regular pulsations. The posterior division 

 VOL. II S 



