ANNULOSA. 1 1 3 



tive appendages are usually present, but are never jointed or articu- 

 lated to the body. 



III. Aetiiropoda. — This division includes the Crustaceans {Crus- 

 tacea), the Spiders, Scorpions, &c. {Arachnida), the Centipedes and 

 their allies {iryriapoda), and the Insects (Tnsecta). The body (fig. 

 71) is composed of a series (usually definite) of distinct rings or 



Kg- 71. — Diagram of the anatomy of an Insect. ttTt Antennse ; e Eye ; m Moutli ; 

 g Gullet ; sg Salivary gland ; s Stoinacli ; / Tubes supposed to represent the kid- 

 neys ; i Intestine ; c Chamber (cloaca) into wliich the intestine opens ; v Vent ; 

 li Heart ; n Nervous system ; I Bases of the legs. 



" somites," arranged along a longitudinal axis. A true blood-vas- 

 cular system is normally present, and the heart is placed dorsally. 

 The nervous system consists primitively of a double chain of ganglia, 

 placed ventrally, and traversed anteriorly by the oesophagus. Limbs 

 are almost always present, and are jointed and articulated to the 

 body. The integument is more or less extensively hardened by 

 the deposition in it of chitine, with or without salts of lime ; and 

 ciliated epithelium is not developed. 



SCOLECIDA. 



The Scolecida are characterised, as compared with the higher 

 Annulose Animals, by their unsegmented or imperfectly segraented 

 bodies ; hy the general absence of lateral appendages ; by the posses- 

 sion of branched water-vessels, filled with a watery fluid, and usually 

 communicating with the exterior ; and hy the fact thai the nervous 

 system consists of one or two ganglia situated in the anterior part of 

 the body. 



A large number of the Scolecids are internal parasites, and are 

 commonly known by the name of Entozoa. Many forms, how- 

 ever, lead a, permanently free existence. The Scolecida are often 

 divided into two great sections — viz., (1) the Platyelmia or Flat- 

 worms, including the Tape - worms ( Tceniada), the Fluke - worms 

 iTrematoda), and the Turbellarian "Worms ; and (2) the Kematelmia 



