310 THE INVERTEBRATA 



somite are the deutocerebrum or mesocerebrum ; those of the third 

 somite are the tritocerebrum or metacerebrum. The identity of some of 

 these gangha may be lost, even in development. Concerning the 

 functions of the central nervous system something is said on p. 448. 

 The eyes of the Onychophora are a pair of simple, closed vesicles, 

 each with its hinder wall thickened and pigmented and its cavity oc- 

 cupied by a lens secreted by the wall. The eyes of all other arthropods 

 (Fig. 211) consist of one or more units each of which is in essence a 

 cup, or a vertical bundle, of cells, over which the cuticle of the body 

 forms a lens. The cells which compose the bottom of each cup are 

 (except in the median eye of the Crustacea) arranged in a sheaf or 

 sheaves called retinulae; in the midst of each retinula is a vertical rod, 

 known as the rhabdom, secreted by the cells of the sheaf in vertical 

 sections which, when they are distinct, are known as rhabdomeres. 

 Each bundle-unit has one such retinula. Sometimes in the cups the 

 retinulae are surrounded by cells which bear on their free ends short 

 rods of the same nature as the rhabdomeres. The retinula cells contain 

 pigment and there is a ring of strongly pigmented cells around the 

 cup. The eye units occur {a) as single cups each with several retinulae 

 (ocelli of insects. Fig. 211 C"), {b) as groups of similar cups placed 

 contiguously (eyes of myriapods), {c) as eyes composed of a number 

 of small cups, each with a single retinula, united together (lateral eyes 

 of Limulus), {d) as true compound eyes (Fig. 212) composed of a 

 number of bundles of cells, each bundle {ommatidium) complex in 

 structure and containing two or more refractive bodies, but each 

 probably representing a narrowed and deepened cup. Compound 

 eyes of this type are found in crustaceans and insects. They vary much 

 in detail, but essentially the structure of an ommatidium is as follows 

 (Fig. 211 D). At its outer end is a transparent portion of the general 

 cuticle of the body, usually thickened to form for the ommatidium a 

 biconvex lens. Under this lie the epidermal cells which secrete it 

 (corneagen cells): the lens is one of the facets of the eye. Under the 

 corneagen cells comes a bundle of two to five vitrellae or crystal cells, 

 grouped around a refractive body, the crystalline cone, which they 

 have secreted. The vitrellae taper inwards and their apex is clasped 

 by a second bundle of cells, four to eight in number, which together 

 form the retinula. Like the vitrellae the retinular cells secrete in the 

 axis of the ommatidium a refractive body. This is the rhabdom, and 

 is made up of rhabdomeres, one for each of the cells. Each retinular 

 cell passes at its base into a nerve fibre which pierces the basement 

 membrane of the eye and enters the optic ganglia. Around each 

 ommatidium, separating it from its neighbours, there are usually 

 pigmented cells, known as iris cells. The eyes of arachnids, other than 

 the lateral eyes of Limulus, simulate the ocelli of insects, but are 



