gg LAMELLIBRANCHIA. 



of these problems seems all the more desirable as the eye of Pecten* in its 

 structure stauds almost alone among Molluscan eyes. With regard to their 

 morphological interpretation, we are inclined to agree with Butschm (No. 

 7) who showed how the pigment cell-layer of the posterior wall of the eye 

 passes over into the retina, a closed vesicle being thus formed in the eye its 

 anterior wall consisting of the retina and its posterior wall of the pigmented 

 integument. This vesicle must be supposed to have arisen by invagination 

 and abstraction from the ectoderm, a view with which Patten's observation 

 of a solid ingrowth can be reconciled. The description given by Patten also 

 of the rise of the lens outside of the optic vesicle supports such a condition it we 

 do not assume a mesodermal origin for the lens but rather imagine a second 

 process of invagination such as occurs in the Cephalopodan eye. The 

 the lens outside of the optic vesicle makes it possible for the more superficial 

 wall of the latter to be changed into the retina, a change which is impossible 

 where the lens has itself arisen from this outer wall, as is the case m the 

 Gastropoda and in some of the Cephalopoda also. The position of the rods 

 is hereby explained (Butschli). Since these always arise at the free ends 

 of the cells, they are directed forward when the deeper wall of the optic- 

 vesicle is transformed into the retina (Gastropoda, Cephalopoda) ; but are, 

 on the contrary, directed backward when the retina is derived from the outer 

 or superficial wall of the vesicle. The latter must originally have been the 

 case in Pecten. 



The otocysts arise, in Teredo and Anodonta, near the pedal gang- 

 lion as invaginations of the ectoderm which then become abstracted 

 from the latter and provided with otoliths and sensory hairs (Fig. 18, 

 U p 36) In Cyclas, the otocysts lie at the two sides of the embryo, 

 behind the lateral end of the ciliated area. [In the Protobranchia the 

 Oocysts retain their connection with the exterior throughout life.] 



Spengel's olfactory organs and the abdominal sensory organs 

 (Thiele) show, by their structure, that they are mere modifications 

 of the body-epithelium. 



D. The Alimentary Canal. 



The structure of the alimentary canal, being greatly influenced by 

 adaptation to different conditions of life, varies in certain points in 

 the different forms. In Odrea, for example, the archenteron is said 

 to pass over direct into the definitive intestine the blastopore remain- 

 iJ open, while in Teredo, as well as in Cyclas and the Unwnulae, the 

 blastopore closes and a true stomodaeum forms. This condition, and 



:^^^S^=i = =^ altogeker 

 different ways. 



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