SUB-KINGDOM IV.ANNULOSA. 

 CHAPTER XII. 



AN ARTHROPOD A. 



SUB-KINGDOM ANNULOSA. In this sub-kingdom are com- 

 prised an enormous number of animals which agree in the 

 following characters (Fig. 44) : The body is composed of a 

 number of segments or rings arranged along a longitudinal 

 axis. There is a distinct alimentary canal (#), placed cen- 

 trally as compared with the other organic systems, and com- 

 pletely shut off from the general cavity of the body. The 



FIG. 44. Diagram of an Aunulose animal, a Blood- vascular or hsomal system; & Digestive 

 system ; c Neural system. 



blood-vascular system may be absent, but, when present, it is 

 always situated on the dorsal aspect of the body (a). The 

 nervous system is always present, and is placed along the 

 ventral surface of the body. In its typical form it consists 

 of two nervous cords running along the whole length of the 

 ventral surface, and having a pair of ganglia developed in 

 each ring. The first pair of ganglia is always placed above 

 the gullet, and the second below, so that the gullet is sur- 

 rounded by the two cords uniting these ganglia (constituting 

 the so-called cesophageal collar). The limbs (when present) 

 are always turned toward the neural aspect that is to say,, 



