THE CH^TOGNATHA, OR PRIMITIVE MOLLUSCA. 373 



It seems to be the homologue of the velum and the rhino- 

 phoral organs enclosed within it of the Mollusc, for the 

 innervation from the cerebral ganglion by two nerves lying 

 just within the optic nerves, is precisely similar. It is 

 possible that the minute organs which Weiss has described 

 in certain Oigopsid Cephalopoda are to be compared with 

 them. And, in this connection, both the tentacles of 

 Spadella cephaloptera and the olfactory spoon-shaped 

 organs of Chiroteuthis should be made objects of further 

 study. 



The eyes are undoubtedly of the Molluscan vesicular type. 

 The retinal cells contain rods; the crystalline lens is secreted 

 within an invagination of the ectoderm, which has become 

 overgrown by a flattened epithelium. The fact that in the 

 Molluscan eye the pigment forms a complete cup to the 

 retina, but is mainly restricted to one side of the ocellus in 

 Sagitta, is a minor point resulting from the peculiar method 

 of the grouping of ChEetognath eyes in threes. 



As in the Amphineura, otocysts are unknown. 



Alimentary Canal. 



If our hypothesis of the origin of the Mollusca be correct, 

 it follows that the straight and symmetrical alimentary canal 

 of the Chsetognatha is in a more primitive condition than 

 that of any other Mollusc. Excepting for the absence of the 

 radula, an organ better adapted to quiet browsing than to 

 predatory Cheetognath habits, the buccal armature is singu- 

 larly like the armature which predatory Molluscs develop. 

 The resemblance both in position and structure between the 

 lateral groups of mandibles of a Gymnosomatous Pteropod, 

 such as Clio (text-fig. 7, h.) and the '^hooks" of Sagitta, is very 

 close. No chitinous teeth are developed in the mid-ventral 

 line, but the repeated series of spines in marginal and lateral 

 groups above the entrance to the buccal cavity suggests a 

 possibility of the successional development and replacement 



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