Introdtiction to Animal Morphology . 327 



CHAPTER XXXIX. 



SUB-KINGDOM 6. — ARTHROPOD A (z'. Siehold). 



Symmetrical, mostly dioecious,* non-ciliated schizo- 

 coelous (p. 47) personae, of a limitedf specifically con- 

 stant number of often heteronomous metameres or 

 somites, each usually with a pair of ventrally-articu- 

 lated,Jhollow,§jointedorgans,asfeelers,jaws, orlimbs. 

 The body consists of head, thorax, and abdomen : the 

 first contains not fewer than four united somites, 

 bearing the sense-organs preorally ; the second bears 

 the locomotory limbs ; the third contains the vegeta- 

 tive and reproductive organs. || The heart, when pre- 

 sent, is dorsal, tubular, often segmented, and the 

 circulation is more or less lacunary. Breathing 

 takes place by the surface, gills, or tracheae. The 

 digestive system is absent in some parasitic Crusta- 

 ceans, aproctous in the larvae of Myrmeleo and some 

 Hymenoptera. In others the mouth is anterior, ven- 

 tral, the anus rarely dorsal (Dias, &:c.), usually termi- 

 nal, or sub-terminal. The intestine is seldom tortuous 

 (Insecta), or it may have lateral caeca (Arachnida). 

 On the surface is a firm, coloured, lamellated, nearly 

 structureless layer of Chitin,1| with or without an in- 



* Except Cirripedes and Tardigrades, and anomalously in some insects. 



t Except in Mpiapods. 



X The wings of insects and the shell-flaps of Entomostraca are unjointed 

 processes, not true limbs. 



\ Containing muscles for their motion. 



II These organs are never metamerically multiplied, though the respira- 

 tory and nervous systems are so arranged. 



IT Pf/zVoif supposes that Chitin (Ci7H;4XOu) is a mixture ofaproteid 

 and a cellulose compound. 



