106 



STRUCTURE OF ARACHNIDA AND CRUSTACEA. 



tained in these cavities. At c t is seen the cephalo-thorax 

 opened from below, and giving attachment to the legs ; at m 

 is shown the place of the mandibles or jaws ; at p is seen one 



pa ab pa s 



p pa t a I s ma o 



Fig. 46. ANATOMY OF SPIDER. 



of the palpi, which are appendages to the mouth ; pa is the 

 foremost leg ; t, the large nervous mass, from which the legs 

 are supplied; a, the collection of ganglia supplying the 

 abdomen ; a 6, the abdomen ; p a, the respiratory chambers ; 

 s s, the stigmata or openings into these ; Z, the leaf-like folds 

 within them ( 323) ; m a, the muscles of the abdomen ; a n, 

 the termination of the intestine ; /, the spinnerets ; o, the 

 ovaries ; and o r, the opening of the oviduct. 



99. The class of CRUSTACEA, of which the Crab, Lobster, 

 and Cray-fifth are the best-known forms, differs from both 

 the preceding, in being adapted to breathe by means of gills, 

 and thus to reside in or near water, instead of inhabiting 

 the air. Moreover, the body is inclosed in a hard covering, 

 which generally contains a good deal of carbonate of lime, and 

 which is thrown off at regular intervals. This covering also 

 incloses the members, which are never less than ten in 

 number, and are frequently more numerous. There is great 

 variety of form among the animals of this group, which is 

 altogether one of great interest. In the Crab tribe, the head, 

 thorax, and abdomen are all drawn together, as it were, into 

 one mass and the general arrangement of the organs it con- 

 tains is exhibited in the succeeding figure, which shows them 

 nearly as they are found to lie, when the upper part of the 

 shell, or carapace, is removed. At t there is left a portion of 



