THE ARTHROPODA 91 



consider a large division of the animal kingdom, which pre- 

 sents many analogies with the polychaete worms the phylum 

 Arthropoda^ or, as some prefer to call it, the Gnathopoda. 

 The Arthropods include the spiders, scorpions, centipedes, 

 insects, water-fleas, sand-hoppers, wood-lice, lobsters, and crabs, 

 and also a peculiar and very interesting worm-like animal 

 found in warm countries in very different parts of the world 

 at the Cape of Good Hope, in New Zealand, New Guinea, and 

 the West Indies, and known to science as Peripatus. 



The Arthropods exhibit a well-marked external segmenta- 

 tion, but internally they are not divided up into compartments 

 as obviously as are the chaetopod worms. Like the latter, 

 they have an external chitinous cuticle, which is frequently 

 thickened locally to form a series of hard rings or anmili, 

 joined together by thinner intervening tracts of the integument, 

 and freely movable on one another. Every such annulus may 

 be, and some always are, provided with a pair of jointed ap- 

 pendages moved by internal muscles attached to the thickened 

 external cuticle which serves as the skeleton of these limbs. 

 It is from these hollow-jointed limbs that the name Arthropoda 

 (apOpos, a joint ; TTOVS, a foot) is derived, and the name Gnatho- 

 poda (yvdOos, a jaw ; irovs, a foot) refers to the feature, char- 

 acteristic of the whole group, that a certain number of these 

 limbs are turned forwards in the region of the mouth, and 

 serve as functional jaws. Such "jaws" work from side to 

 side, and must not be confounded with the jaws of polychsete 

 worms, which similarly work from side to side, but are only 

 local thickenings of the integument of the lips ; nor with the 

 jaws of vertebrated animals, which work up and down. Further 

 points characteristic of Arthropods are, that all their muscles 

 are transversely striated, that cilia are absent, or very rarely 

 present, and that the ccelom is very much reduced, being 

 replaced by an extensive system of blood-spaces or sinuses. 



The Crustacea is a large class of Arthropoda, comprising 

 the water-fleas, sand-hoppers, barnacles, wood-lice, shrimps, 

 prawns, lobsters, crayfishes, and crabs. These are nearly all 

 aquatic animals, and most of them are marine. Living in the 

 water, they breathe by means of special respiratory outgrowths 

 attached either to the limbs or body-walls and known as gills 

 or branchiaB. The terrestrial forms have generally lost these 

 branchiae and have different kinds of specially constructed 



