ANNULOSA: CRUSTACEA. IQI 



2. ARACHNID A. Respiration by pulmonary vesicles, by trachea, 

 or by the general surface of the body. Head and thorax united 

 into a cephalothorax. Antenna (as such) absent. Legs eight. 

 Abdomen without articulated appendages. 



3. MYRIAPODA. Respiration by trachea; head distinct; 

 remainder of the body composed of nearly similar somites. One 

 pair of antenna. Legs numerous. 



4. INSECTA. Respiration by trachea. Head, thorax, and 

 abdomen distinct. One pair of antenna. Three pairs of legs, 

 borne on the thorax. Abdomen destitute of limbs. Generally two 

 pairs of wings on the thorax. 



CHAPTER XXXI. 

 CRUSTACEA. 



CLASS I. CRUSTACEA. The members of this class are com- 

 monly known as Crabs, Lobsters, Shrimps, King-crabs, Bar- 

 nacles, Acorn-shells, &c. They are nearly allied to the suc- 

 ceeding order of the Arachnida (Spiders and Scorpions) ; but 

 may usually be distinguished by the possession of articulated 

 appendages upon the abdominal segments, by the possession 

 of two pairs of antennae, and by the presence of branchiae. 



" In the Crustacea the body is distinguishable into a variable 

 number of * somites,' or definite segments, each of which may 

 be, and some of which always are, provided with a single pair 

 of articulated appendages. ... In most Crustacea, and 

 probably in all, one or more pairs of appendages are so modi- 

 fied as to subserve manducation. A pair of ganglia is primi- 

 tively developed in each somite, and the gullet passes between 

 two successive pairs of ganglia, as in the Annelida. 



" No trace of a water- vascular system, nor of any vascular 

 system similar to that of the Annelida, is to be found in any 

 Crustacean. All Crustacea which possess definite respiratory 

 organs have branchiae or outward processes of the wall of the 

 body, adapted for respiring air by means of water ; the terres- 

 trial Isopoda, some of which exhibit a curious rudimentary 

 representation of a tracheal system, forming no real exception 

 to this rule. When they are provided with a circulatory organ, 

 it is situated on the opposite side of the alimentary canal to 

 the principal chain of ganglia of the nervous system ; and com- 

 municates by valvular apertures with the surrounding venous 

 sinus the so-called ' pericardium.' " (Huxley.) 



