690 ZOOLOGY SECT. 



connectives, and their fibres are derived from the cerebro-pleural 

 ganglia. 



The osphradia " olfactory " or water-testing organs are patches 

 of sensory epithelium with an accessory or osphradial ganglion 

 situated in immediate relation with the visceral ganglia (Fig. 601, 

 VIII), but connected by nerve-fibres with the cerebro-pleural 

 ganglia. Patches of sensory epithelium, very similar to the 

 osphradia, and called the abdominal sense-organs, occur one on each 

 side of the anus in Area and other forms devoid of siphons, and a 

 similar organ has been described beside the retractor muscles of 

 the siphons in several Sinupalliata. 



In a few instances eyes are present, but never in what we are 

 accustomed to consider as the normal position for such organs, 



B * C 



r 



U rk. s d ' " 



rn.es. 



E. 



6*. 



me!?. 



Fia. 603. Five stages in the development of Ostrca. a. anus ; bl. blastopore ; m. mouth ; ma. 

 stomach; mes. mesoderm ; rk. polar bodies ; s. shell ; sd. shell -gland ; sm. anterior adductor ; 

 w. pre-oral circlet of cilia. (From Korschelt and Heider.) 



at the anterior or head-end of the body. They occur, in fact, in 

 the only situation where they can be of any use, namely, along the 

 edge of the mantle. The best-known form in which they occur is 

 the common Scallop (Pecteri), which has a single row (Fig. 585, VII) 

 all round the mantle-border. Each has a cornea (Fig. 602, I), a 

 cellular (not cuticular) lens (2), a retina (5) formed of cells, the 

 inner ends of which are modified into visual rods, and an optic 

 nerve (7), one branch of which spreads over the front of the retina 

 and sends branches backwards to the visual rods. In this 

 peculiarity, as well as in the cellular lens, the eye of Pecten is 

 singularly like that of Vertebrates. The pallial eyes of Pelecypoda 

 are probably to be looked upon as modified tentacles. The only 

 cephalic eyes that occur in this class are a pair of small but well- 



