1887.] 



ON CYCLEHESTERIA HISLOPI. 



30 



faculty being apparently restricted to the above-mentioned recurved 

 spines, the delicate structure of which, in conjunction with the 

 numerous nerve-cells crowded at their bases, must serve to corro- 

 borate such an assumption. 



The antennulæ, on the other hand, cannot, I think, be re- 

 garded as true tactile organs, since they never admit of being 

 exserted beyond the edges of the shell. In all probability, they 

 will prove, as generally admitted with serveral other crustaceans, 

 olfadory in function, the peculiar and very delicate structure 

 of the apical sensory filaments seeming also to indicate a more 

 specialized sense, different from that manifested by the an- 

 tennæ. 



As to the compound eye, its function as a true visnal organ 

 cannot of course be disputed. Somewhat more doubtful, it would 

 seem, is the function of the so-called ocellus, the structure of 

 this organ being, as above stated, very different from that of the 

 compound eye. But, remembering that also in higher Arthropoda, 

 for example insects, there are often two very different kinds of 

 visual organs in the same animal, viz., the faceted and the simple 

 eyes, and that in the larvæ of other Phyllopoda compound eyes 

 are not developed till very late, whereas the ocellus is present 

 in the earliest stages, we may, it would seem, be warranted in 

 ascribing to this organ a true visual function, though the vision 

 by the aid of that organ in all probability is of a somewhat 

 different character to that resulting from the compound eyes. 



Some naturalists have regarded the ocellus as a mere provi- 

 sional organ, function ing only in the larvæ, and becoming in 

 the adult animal more or less obliterated or degenerated. 

 This assumption cannot, however, be maintained as regards the 

 present form, in which this organ is very fully developed in the 

 adult animal, being scarcely at all smaller than the compound eye. 

 To the ocellus must be ascribed, in my opinion, the function of 

 a coadjutory or supplementary visual organ, and, as apparently 

 supporting such an assumption, may, perhaps, in the case of the 

 present form be adduced the fact, that, while the greater part 

 of the crystalline cones of the compound eye point superiorly and 



