Introduction to A nimal Morphology. 327 



CHAPTER XXXIX. 



SUB-KINGDOM 6. ARTHROPODA (v. Stcbold}. 



SYMMETRICAL, mostly dioecious,* non-ciliated schizo- 

 ccelous (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, hollow,^ jointed organs, as feelers, jaws, or limbs. 

 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 (Bias, &c.), usually termi- 

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

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

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

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



* Except Cirripcdcs and Tardigradcs, and anomalously in some insects. 

 t K\cc; t 1:1 M 



'. the shell-flaps <>1 :e unjointed 



' 

 || Tl. 'v multiplied, thou-h the i 



* ' ; ) is a mixture of a protcid 

 ind. 



