i6o 



THE PINEAL ORGAN 



wall of the vesicle is differentiated into a retina of the same type as in 

 Trochus and Patella, consisting of well-defined bacillary, pigment, and 

 nuclear layers. Within the cavity of the optic vesicle the contents have 

 become differentiated into a more solid refractile body, the lens, and a 

 more fluid part which surrounds the lens and, like the vitreous of the 

 vertebrate eye, intervenes between the lens and the retina. Both lens and 

 vitreous are formed by modification of the secretion of the cells lining the 

 vesicle, and a lens of this type is known as a vitreous or non-cellular lens. 

 The development of gasteropods has been worked out by Patten, 

 Boutan, and others, more especially in Patella and Acmcea, a nearly allied 

 species. A typical trochophore larva is formed with an apical plate 

 bearing a vertical tuft of cilia, beneath which there is developed from 

 the ectoderm a cerebral ganglion (Fig. 114, p. 156). The ocelli are de- 

 veloped from the same ectodermal layer, just behind and lateral to the 

 bases of the tentacular processes, and are well seen in the veliger larva of 

 Acmcea virginea (Fig. 118, A). At a later stage the velum or prototroch 

 is shed and the eyes are covered by the anterior margin of the shell (Fig. 

 118, B). The relation of the ocelli to tentacles, velum, mouth, statocysts, 

 and foot are well shown in Fig. 109, p. 150), showing the veliger stage of 

 development in Ver metes. This is a worm-like gasteropod characterized 



ret m 



ft v ot 



Fig. 118. — Advanced Stages of Development of Acjvuea virginea. A: 

 Lateral View of Veliger Larva. B : Dorsal View of Young Acmjea, 



AFTER THE VELUM HAS BEEN CAST 



an. : anus. 



ft. : foot. 



gl. : glands in roof of mantle. 



int. : intestine. 



oc. : ocellus. 



op. : operculum. 



After the velum has been cast off the young mollusc sinks to the bottom ; 

 the periostome enlarges and gives rise to the conical adult shell, the margin 

 of which overlaps the eyes. 



(After Boutan — redrawn from MacBride.) 



