198 



THE CRAYFISH. PHYLUM ARTHROPODA 



the development of the crayfish and of related animals that the 

 foremost region of the head corresponds to a somite, we shall 

 regard the body as containing twenty segments, of which the fore- 

 most bears no Hmbs. The telson is not a segment. Of the twenty 

 segments, the first six form the head, the next eight the thorax, 

 and the last six, with the telson, the abdomen. The parts of which 

 a complete limb consists are best seen in the hmbs known as 

 the third pair of maxillipeds (Fig. 133, H), which lie immediately 

 in front of the great pincers. This iUustrates fairly weU the type of 

 crustacean appendage called the biramous limb or stenopodium, 

 which has a base or protopodite, two branches called endopodite 

 and exopodite, and sometimes extra processes or epipodites. The 



Fig. 133 H. — Third maxilliped ; the first walking leg of a crayfish. 



other limbs are built upon the same general plan as the third 

 maxilhped, but they have modifications suited to their functions, 

 and parts are often missing. They are shown in Fig. 133 F and G 

 and Table L More primitive Crustacea have a flattened type of 

 limb called the phyllopodium. 



CUTICLE AND EPIDERMIS 



The ectoderm or epidermis of the crayfish consists of a layer 

 of protoplasm with nuclei, which in many parts is not divided 

 into cells and is therefore a syncytium (p. 35), though in places 

 it forms a columnar epithelium. Outside it lies a cuticle which 

 it secretes, which contains chitin (p. 159), and is for the most 

 part thick and hardened with salts of lime, but remains thin and 

 flexible in certain places so as to form joints which allow the 

 parts of the body to move upon one another, and also in the 

 gill chambers. In places it bears bristles (setae) of various shapes. 



