ANNULOSA: CRUSTACEA. 20I 



" 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 Cmstacea, 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.) 



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 

 is generally from five to seven. They all pass through a series 

 of metamorphoses before attaining their adult condition, and 

 every part that is found in an embryonic form, even though 

 only temporarily developed, may be represented in a permanent 

 condition in some member of a lower order. 



The classification of the Crustacea is extremely complicated, 

 and hardly any two writers adhere to the same arrangement. 

 The tabular view which follows embodies the arrangement 

 which appears to be most generally adopted, and the diagnostic 

 characters of each order will be briefly given, a more detailed 

 description being reserved for the more' important divisions of 

 the class. Before proceeding further, however, it will be as well 

 to give a description of the morphology of a typical Crustacean, 

 selectirig the Lobster as being as good an example as any. 

 , The body of a typical Crustacean may be divided into three 

 regions — a head, a thorax, and an abdomen, each of which is 

 composed of a certain number of somites, though opinions differ 

 both as to the number of segments in each region, and as 

 to their number collectively. By the majority of writers the 

 body is looked upon as being typically composed of twenty-one 

 segments, of which seven belong to the head, seven to the 

 thorax, and seven to the abdomen. In many Crustacea, how- 



