94 COMPARATIVE ANATOMY. 



received the name of the 'pre-pigmental retina,' though no 

 connection between it and the optic nerve has been demon- 

 strated. The crystalline lens is unique in being divided trans- 

 versely into two parts an anterior and a posterior between 

 which a fold of choroid membrane is inserted. The posterior 

 segment is much larger than the anterior, and projects back- 

 ward into the interior of the eye. At the posterior part of 

 the orbit a closed cavity is limited in front by the eyeball, and 

 behind by a layer of the sclerotica, which, extending back- 

 ward, encloses the terminal portion of the optic nerve and a 

 gangliform enlargement upon its trunk. Between the globe 

 of the eye and cornea is seen a chamber resembling a serous 

 cavity, but which communicates with the external medium 

 by a minute central opening. Its lining membrane is analo- 

 gous to that of a conjunctiva! surface. 



ARTICULATA. Annelida. Simple eyes not unfrequent in 

 the dorsibranchiates. For eyes of Hirudo (leech), see p. 159. 

 Remarkably large form in Torrea. 



Crustacea. Eyes agglomerate and extremely brilliant in 

 Daphne. To a mass of ocelli, often chiefly composed of P3 7 ri- 

 form lenses, a single cornea is found. Compound aggregated 

 eye, either sessile or pedunculated, seen in Decapoda, as 

 Homarus (lobster). Fine illustrations of compound eye met 

 with in extinct trilobites. 



Cirripedia (barnacles) Eyes multiple ; remarkable for 

 being concealed within abdomen rays of light transmitted 

 through the tissues. 



Insecta. Eyes single or compound, aggregated. "When 

 compound, each ocellus possesses a flattened cornea, a pris- 

 matic vitreous humor, and between the two a layer of vari- 

 ously-colored pigment matter. One optic nerve supplies all 

 the ocelli of a mass, each ocellus being furnished with a dis- 

 tinct branch from the main trunk; the so-called 'retina* is 

 an expansion of nerve-like substance between the secondary 

 and terminal subdivisions of the nerve. 



The ocelli in the majority of insects are arranged in two 

 lateral masses, one on either side of the head, each mass 

 being made up of large numbers of eyes. In Musca (house- 



