THE VELIGER LARVA. 133 



us the tentacles (Fig. 55, /). At their bases, the eyes (a) arise. 

 Both the tentacles and the eyes are, by their origin, indisputably 

 proved to belong t<> the primary cephalic section, and it is of special 

 interest that the tentacles occupy the same position as the cephalic 

 tentacles of the Annelida and the Annelidan larvae (Vol. i., Figs. 120 

 B, p. 269, and 121, p. 270). 



The velum may still persist after the foot has attained a consider- 

 able size and when the development of the other organs also is far 

 advanced, but it gradually diminishes in size and finally degenerates, 

 the larva thereby passing over to the adult condition which, indeed, 

 had already been nearly approached. Two small rounded ciliated lobes 

 may pei'sist near the mouth as the remains of the velum, as was 

 observed by Kay Lankester in Limnaea (No. 63), and Joyeux- 

 Laffuie in Onchidium (No. 51). These are said to give rise to the 

 sub-tentacular lobes or lip-tentacles ; these two structures would thus 

 have an origin similar to that which we felt inclined to assume for 

 the oral lobes of the Lamellibranchs (p. 45). 



The perfectly developed Veliger larva is found almost exclusively 

 among the marine Gastropoda, the young of which swim about freely 

 for a long time. Among fresh-water Gastropods, Neritina passes 

 through a stage with a well-developed bilobed velum resembling that 

 depicted in Fig. 55 ( Vermetus), but the Veliger larva does not lead a 

 free life, but passes through this stage within the egg-capsule. When 

 the embryo leaves the capsule it shows the adult form (Claparede, 

 No. 23). Neritina is one of those fresh-water forms which can also 

 live in salt water. This fact, and the presence of the well-developed 

 I r eliger stage, suggest that it has only recently adopted a fresh-water 

 existence. In other fresh- water Prosobranchs, as well as in aquatic 

 and terrestrial Puhnonates, the Veliger stage is much reduced. 

 Onchidium, however, among the Pulmonates, in this respect resembles 

 Neritina. 



Onchidium, a Pulmonate living between tide-marks, not only passes 

 through a TrocJiophore stage but, while still within the egg-shell, 

 becomes a Veliger larva with coiled shell and a large bilobed velum. 

 In the course of further development, the velum degenerates, only 

 two rounded lobes which lie laterally to and somewhat in front of the 

 mouth being retained as the lip-tentacles. The embryo, on leaving 

 the eug-shell has, on the whole, the same shape as the parent 

 (Joyeux-Laffuie, No. 15). The condition we have just described 

 would be very remarkable in a Pulmonate, did not the organisation and 

 the manner of life of this form give some cause for the assumption 



