COMPOUND EYES OF ARTHROPODS. 21tl 



The cryBtalline cone is purely a dioptric- appiiratui^. A careful 

 study of its structure, both in the adult and in tiio youn- eir.bryos 

 oi Serolis,\v.i& clearly shown that this has nothing' to do with the 

 sensory function, as it has no connection whatever with the optic 

 nerve fibres. 



The vitrella, then, agrees with the corneagen cell in the 

 capacity for secreting chitin on one portion of its surtace. The 

 diflerence between them consists in this, that while the corncagen 

 cells secrete chitin towards the exterior of the body, the vitrellae 

 secrete it towards each other, that is, towards tiie median axis of 

 the omniatidium. 



Next below the vitrella comes the important stratum {c,Y\g.\a) 

 which lies at the bottom of the whole structure, and which sends 

 processes inward {Op. n) to form the optic nerve fibre, which 

 terminates in the optic ganglion. 



The cells in this structure are also characterized by secreting 

 chitin towards the median axis of the omniatidium. The chitin 

 thus produced invests the colorless portion of the cell (c) consti- 

 tuting the rod or the rhahdomere (Lankester and Bourne).^ 



A large number of transverse striae, running at right angles to 

 the longitudinal axis of the ommatidium, are seen through the 

 transparent cuticle. The cell itself which gives rise to the 

 rhabdomere has been known as retinula (Grenacher).' The 



Phacops Rana, Oreen, Journal of Morphology, Vol. II, No. 2, 1888) show, so 

 far as I understand his account, that the lens has a somewhat similar structure 

 to the crystalline cone of SeroUs, or, in fact, to the crystalline cone of Isopod 

 Crustacea, as far as it is known. From Clarice's description I gather that the 

 lens of Phacops is unequally biconvex., the curvature being the greatest on the 

 proximal surface ; and that the lens was hollow, probably filled with some viscid 

 humor. In the absence of a more complete knowledge on the nature of a corneal 

 covering to the eye in the Trilobite it is premature to carry out the homology of 

 the " crystalline cone" of Serolis and of the Isopods in general to the " lens" 

 of a Trilobite, although there appears to exist a certain resemblance between the 

 two. It is perhaps worthy of mention, in this connection, that there is nothing 

 whatever in the lens-cone of Limulus at any period of its development wliich 

 shows any resemblance to the lens of a Trilobite as made out by Clarke. The 

 lens-cone of Limulus is the solid, conical projection of the chitinous cuticle of the 



body, fitting into the open depression of the skin, and in no period of the life 



history of the animal is it separated from the outermost cuticle of the body. 



(Compare Grenadier, Lankester and Bourne, and Part III of the present paper 



on the compound eye of Limulus.) 

 • loc. cit. p. 183. 

 > Untersuchungen uber das Sehorgan der Arthropoden, 1879. 



