ARTHROPODA 309 



used in feeding, while the divisions, if any, of the hinder part of the 

 body {opisthosoma or "abdomen") are known as the mesosoma and 

 metasoma. It is important that the student should recognize that each 

 of these divisions varies in size, and that consequently none of them 

 comprises in all arthropods the same somites, so that, for instance, 

 the thorax of an insect is a quite different entity from that of a cray- 

 fish. The most significant variation is that of the head, which, as the 

 organization of its possessor becomes higher, increases in size, taking 

 in behind somites whose appendages become jaws, while, by altera- 

 tion in the position of the mouth, it adds others, whose limbs become 

 antennae, to its preoral sensory complex. Thus, while the head of the 

 Onychophora comprises only the first three somites, and only the first 

 of these is preoral, in the Crustacea there are in the true head six 

 somites (including the embryonic first somite), of which three are 

 preoral, and thoracic somites, whose limbs (maxillipeds) function as 

 jaws, are often united with the head. 



The paired limbs of arthropods present an enormous variety of 

 form, and attempts have been made to reduce them to a common type. 

 Some of the evidence suggests an archetype with a nine-segmented 

 axis bearing on the median side of the first segment a biting process 

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

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

 and the problem is still far from solution. 



The arthropod cuticle has a thin, impermeable, non-chitinous 

 external layer (epicuticle) and a thick, elastic, permeable, lamellar 

 inner layer (endoctiticle),\sLrge\y composed of chitin ^, the outer lamellae 

 usually hardened, often by salts of lime. From time to time during 

 the growth of the animal, the hard outer layers of the cuticle are 

 separated by solution of the inner layers by an enzyme, ruptured, 

 and shed in a moult or ecdysis. A new cuticle which has formed 

 under it then expands 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 &ense organs (eyes, frontal organs), 

 and sometimes also of a median anterior element {archicerebrum, 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 



^ Chitin is an amino-polysaccharide which resists most solvent agents. 



