274 THE INVERTEBRATA 



(gnathobase) and on a more distal segment an outer branch (exopo- 

 dite) ; but there are difficuhies in the way of assuming this in all cases, 

 and the problem is still far from solution. 



The arthropod cuticle is composed of chitin, often hardened by a 

 deposition of salts of lime. From time to time during the growth of 

 the animal, the hard outer layers of the cuticle are separated from the 

 inner layers, ruptured, and shed in a moult or ecdysis. The soft inner 

 layers then expand to accommodate the body. 



The nervous system of arthropods contains, in typical instances, on 

 two longitudinal ventral cords and in a dorsal brain, a pair of 

 ganglia for each somite, but where the somites are fused there is often 

 a fusion of their ganglia, and where they bear no limbs their ganglia 

 may be absent. The brain is a complex structure composed of the 

 ganglia of the somites which have become preoral (though in a few 

 Crustacea the antennal ganglion remains postoral), of paired ganglia 

 for certain primitively preoral presegmental sense organs (eyes, 

 frontal organs), and sometimes also of a median anterior element 

 {archicerebruMy in the strict sense). The ganglia of the first somite 

 are known as the protocerebrum ; with the ganglia anterior to them 

 they constitute the procerebrum {archicerebrum of Lankester). The 

 ganglia of the second somite are the deutocerebrum or mesocerebrum ; 

 those of the third somite are the tritocerebrum or metacerebrum. The 

 identity of some of these ganglia may be lost, even in development. 



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

 with its hinder wall thickened and pigmented and its cavity occupied by 

 a lens secreted by the wall. The eyes of all other arthropods (Fig. 200) 

 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. 200 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. 201) com- 

 posed of a number of bundles of cells, each bundle (ommatidium) 

 complex in structure and containing two or more refractive bodies, but 



