AXXULOSA. 



113 



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

 lated to the body. 



III. Akthropoda. — This division includes the Crustaceans (Crus- 

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

 their allies (Jfyriapoda), and the Insects {Insecta). The body (fig. 

 71) is composed of a series (usually definite) of distinct ring.s or 



Fig. 71. — Diagram of tlit? aiiatniny of an Insect, an Anteium- ; c Eye ; m Mouth ; 

 ff Gullet; sg Salivary gland; s Stomaeli ; /Tubes sui>iioHe(i ti) repre.sent tlie kid- 

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

 h Heart ; n Nervous system ; I B.ises of the legs. 



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

 cular system is normally present, and the heart ii ])laced dorsally. 

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

 placed ventrally, and traversed anteriorly by the cesophagus. Limbs 

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

 b:.idy. The integument is more or less exten.si\'e!y hardened by 

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

 ciliated epithelium is not developed. 



JOLECIDA. 



The Svoleeida are characterised, as compared with the higher 

 Annulose Animals, by their iiiisegmeiited or imperfectly segmented 

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

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

 communicating %oith the exterior ; and by the fact that the nervous 

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

 the body. 



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

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

 ever, lead a permanently fiee existence. The Scolecida ai'e often 

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

 worms, including the Tape - worms {Teemada), the Fluke - worms 

 ( Trematoda), and the Turbellarian Worms ; and (2) the Nematelmia 



