218 SYSTEMATIC ZOOLOGY. 



The excretory organs of the arthropod are formed upon 

 two plans. In the one (Crustacea and Acerata) we have 

 a few organs, like the green gland and the shell gland, 

 which are apparently to be regarded as comparable to 

 the nephridia of the annelids (p. 184). The other type, 

 the Malpighian tubes (Arachnida and Insecta) are con- 

 nected with the alimentary tract. 



The Arthropoda are by far the largest group of animals, 

 the number of forms living to-day being estimated from 

 half a million to a million or more. 



The Arthropoda are subdivided into three groups or 

 classes: Crustacea, Acerata, and Insecta, and besides 

 these a few forms of uncertain position. 



CLASS I. CRUSTACEA. 



The crayfish and sow-bug may be taken as types of the 

 Crustacea, or crab-like forms. All have two pairs of 

 appendages (antennae) in front of the mouth; they have 

 a varying number of segments at the front of the body, 

 covered by a common shell or carapax, and, excepting 

 gill-less microscopic forms, they all breathe by means of 

 gills attached to some of the feet. 



The number of segments in the body varies; in the higher 

 groups it is constantly twenty, but in the lower it may fall 

 far short of, or far exceed, that number. The regions also 

 vary in extent and cannot be compared throughout the 

 group. Taking the segments connected with the senses 

 and with eating as constituting the head, this region may 

 contain as few as five or as many as eight segments. Not 

 infrequently the head and the next region of the body are 

 united so that they are called a cephaluthorax. The abdo- 

 men is usually well developed, but it may be reduced to a 



