ANNULOSA : CH^TOGNATHA. 



199 



hinder extremity. A single oval ganglion lies in the abdomen, 

 and sends, forwards and backwards, two pairs of lateral cords. 

 The lateral cords unite in front of and above the mouth into a 

 hexagonal ganglion. This gives off two branches which dilate 

 at their extremities into the spheroidal ganglia, on which the 

 darkly pigmented imperfect eyes rest. The ovaries, saccular 

 organs, lie on each side of the intestine and open on either side 

 of the vent ; receptacula seminis are present. Behind the anus, 

 the cavity of the tapering caudal part of the body is partitioned 

 into two compartments; on the lateral parietes of these, cellular 

 masses are developed which become detached, and, floating 

 freely in the compartment, develop into spermatozoa. These 

 escape by spout-like lateral ducts, the dilated bases of which 

 perform the part of vesiculm seminales. The embryos are not 

 ciliated, and undergo no metamorphosis." — (See Introduction to 

 the Classification of Animals, p. 52.) 



CHAPTER XXX. 



ARTHROPODA. 



Division II. Arthropoda, or Articulata. — The remaining 

 members of the sub-kingdom Annulosa are distinguished by 

 the possession oi jointed hppendages, articulated to the body; and 

 they form the second primary division — often called by the 

 name Articulata. As this name, however, has been employed 

 in a wider sense than is understood by it here, it is, perhaps, 

 best to adopt the more modem term Arthropoda. 



The members of this division, comprising the Crustacea 

 (Lobsters, Crabs, &c.), the Arachnida (Spiders and Scorpions), 

 the Myriapoda (Centipedes), and the Insecta, are distinguished 

 as follows : — 



The body (fig. 54) is composed of a series of segments, 

 arranged along a longitudinal axis; each segment, or " somite," 

 occasionally, and some always, being provided with articulated 

 appendages. Both the segmented body and the articulated 

 limbs are more or less completely protected by a chitinous 

 exoskeleton, formed by a hardening of the cuticle. The 

 appendages are hollow, and the muscles are prolonged into 

 their interior. The nervous system in all, at any rate in the 

 embryonic condition, consists of a double chain of ganglia, 

 placed along the ventral surface of the body, united by longi- 



