ANNULOSA: CEUSTACEA. 171 



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

 Abdomen without articulated appendages. 



3. MYRIAPODA. Respiration by trachea?; head distinct ; remain- 

 der of the body composed of nearly similar somites. One pair of 

 antennce. Legs numerous. 



4. INSECTA. Respiration by trachece. Head, thorax, and abdo- 

 men 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 commonly 

 known as Crabs, Lobsters, Shrimps, King-crabs, Barnacles, 

 Acorn-shells, &c. They are nearly allied to the succeeding 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 antennee, 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 ter- 

 restrial 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.) 



In addition to these characters, the body in the Crustacea 

 is always protected by a chitinous or sub-calcareous exoske- 

 leton, or ' crust,' and the number of pairs of articulated limbs 



