176 ' ARTHROPODA [CH. 



vegetation. They have a few pairs of stumpy legs each ending in 

 two claws. They are devoid of antennae, jaws and chelicer-ae. 



In order to get a clear and definite idea of the structure of the 

 Arthropoda we shall select for closer examination the common fresh- 

 water Crayfish which belongs to the class Crustacea. This in England 

 is Astacus fluviatilis, an animal which when full grown is six inches 

 long, but in America the "common" crayfish is represented by 

 various species of the allied genus Cambarus : the common denizen 

 of the St Lawrence is Cambarus virilis, a smaller and more slender 

 creature than the English crayfish, not exceeding five inches in 

 length. The crayfish looked at from above is seen to be composed 

 of a nearly cylindrical anterior portion the cephalo-thorax and 

 a hinder more flattened portion divided by constrictions into a series 

 of similar rings placed one behind the other and at once recalling 

 the segments of an earthworm. This jointed portion is called the 

 abdomen. The cephalo-thorax is covered above by a hard horny 

 skeleton called the carapace (c, fig. 70) and the rings of the 

 abdomen are similarly covered by pieces of a similar skeleton which 

 are called terga. Close inspection shows that a similar though 

 thinner covering extends over the joints between the cephalo-thorax 

 and the abdomen and between the various rings of the abdomen 

 in fact the whole body of the crayfish has a continuous covering 

 of horny matter, thin in some places, thick in others, which is to be 

 regarded as an exaggeration of the cuticle found in the worm. The 

 same is true of all Arthropoda and to this circumstance may be 

 traced directly or indirectly most of the characteristics of the 

 phylum. The thick pieces of the skeleton are known as scle rites 

 and the thin flexible portions which permit of the movement of one 

 sclerite on the other are termed arthrodial membranes. 



Each segment of the abdomen has a pair of outgrowths movably 

 articulated with it called appendages. These differ from the 

 parapodia of Polychaeta chiefly in being flexible only at certain 

 places, and in being on the whole of simpler form. Like the typical 

 parapodium, each consists of a basal piece and two forks, an outer or 

 dorsal called the exopodite and an inner or ventral called the endo- 

 podite. The basal piece is called the pro topodite. As compared 

 with the parapodia of a worm the appendages spring more from the 

 ventral surface and less from the sides, and hence the dorsal branch 

 of the parapodium corresponds to the outer branch of the limb. 



The ventral surface of the segment is protected by a narrow 

 sclerite called the sternum and between successive sterna there are 



