66 LAMELLIBBANCHIA. 



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

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

 morphological interpretation, we are inclined to agree with BUTSCHLI (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 imagination 

 and abstriction 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 if 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 rise of 

 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 in 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 Anodouta, near the pedal gang 

 lion as invaginations of the ectoderm which then become abstricted 

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

 <>t, p. 36). In Cydas, the otocysts lie at the two sides of the embryo, 

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

 otocysts 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 Oatrea, for example, the archenteron is said 

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

 ing open, while in Teredo, as well as in Cyclas and the Unionidae, the 

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



* Spondylus has eyes similar in structure to those of Pecten. The eyes 

 found on the dorsal papillae of Onchidium also resemble those of Pecten 

 in so far as the rods in them are turned away from the light. We thus 

 find similar complicated structures, which must have arisen in altogether 

 different ways. 



