264 COMPARATIVE ANATOMY. 



cells (<j), wliicli at first are placed beside tlie pigment cells, comes 

 next. These cells represent a vitreous body. Lastly, there are the 

 cells (y) which form a kind of retina ; these are connected with the 

 optic nerve (c), but converge towards the exterior, and so towards 

 the posterior surface of the lens, where they undergo various kinds 

 of differentiation. The vitreous body, pigment cells, and " retina u 

 are therefore clearly continuous with the ectodermal layer (hypo- 

 derm), and are differentiations of it, just as the cornea-lens was 

 formed from the cuticular layer, which again can be derived from 

 the hypoderm. The elements which compose the eye undergo 

 various differentiations. A special structure, the " rod," is generally 

 differentiated in the anterior end of the retinal cells. When a 

 number of these cells are united into a single apparatus the rods also 

 become united, and form a special structure, the " rhabdom," in 

 the long axis of a group of combined retinal cells. The retinal cells, 

 which give rise to the rhabdom, constitute a " retinula." The cells 

 in the vitreous body in front of the retinal cells may also undergo 

 great modifications. Each group separates off a transparent highly 

 refractive substance, which forms the so-called "crystalline cone ;" 

 the apex of this is turned towards the rhabdom, and its base towards 

 the integument, i.e. to the cornea-lens. 



Owing to the different development of the various parts, and to 

 the various ways in which they are combined, the optic organ 

 of Arthropoda becomes very varied in chai*acter. Muscular fibres 

 sometimes enter into the composition of the eye, and appear to 

 form a focussing apparatus. 



These eyes belong to the head. The optic nerve arises from 

 the cerebral ganglion. In all divisions the eyes may undergo 

 degeneration, and even completely disappear. Eyes are but seldom 

 developed on other parts of the body, as they often are in the 

 Annelides, so that the presence of eye-like organs on the thorax and 

 abdomen of the Schizopod genus Euphausia is a rare exception. 



§ 205. 



The simplest eyes, although indeed their structure is not yet 

 exactly known, are found in the Eutoinostraca. Each eye appears 

 to possess one crystalline cone only, which is sunk into a mass of 

 pigment, and is generally separated from the integument. Two such 

 eyes, which are generally placed immediately on the cerebrum, are 

 characteristic of the Nauplius form of the Entomostraca. There arc 

 two eyes, connected in the middle line, placed close to one another, 

 and fused into one organ by the connecting pigment ; when they 

 are not placed on the cerebrum itself, they are carried on a median 

 process of it. The Cirripedia and Rhizocephala have eyes during 

 their larval condition ; the latter lose them later on. In many free- 

 living Copepoda the eye is more or less distinctly divided into two. 

 In that case there is another and larger eye on each side, in addition 

 to the larval eye. Each of these is generally provided with a 



