248 MANUAL OF ELEMENTARY ZOOLOGY 



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. 119), 

 though in places it forms a columnar epithelium. 

 Ep*de'™is d Outside it, lies a cuticle which it secretes, and, 

 as we have already seen, this cuticle is for the 

 most part thick and hardened with salts of lime, so as to 

 form an armour, 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 

 many places it bears bristles of various shapes. These are 

 hollow, and the epidermis is continued into them and is 

 here often connected with nerve fibres, so that the bristles 

 serve as sense organs of various kinds. From time to time 

 the cuticle is shed and a new one secreted. This allows of 

 growth and also serves as a form of excretion, for the horny 

 basis of the cuticle, known as chitin, is a complex compound 

 of ammonia, so that in it the animal gets rid of nitrogenous 

 waste matter. Moulting takes place frequently while the 

 animal is growing, but the full-grown male sheds its cuticle 

 only twice a year, and the female only once. As the time 

 for moulting draws near, the crayfish goes into hiding, 

 because the new cuticle is soft and the animal will be 

 helpless for some days while it is hardening. The shell 

 then splits across the back and along the limbs, and the 

 crayfish, lying on its side, draws itself out of the old 

 cuticle. 



There is in the crayfish no continuous muscular body- 

 wall, but numerous muscles, composed of striped 

 Musoto" 'and ^ Dres ' m ° ve the various parts of its body, being 

 Looomot'ion. attached to the inside of the pieces of the armour. 

 Thus the skeleton is external, not, like that of 

 a frog, internal. In the thorax ingrowths of the cuticle 

 provide a kind of false internal skeleton. This has the 

 form of a complicated scaffolding along the ventral side of 

 the animal, and is known as the endophragmal skeleton. 

 Two sets of muscles move the abdomen. A dorsal set of 

 extensor muscles starts from the inside of the carapace and 

 is inserted into the terga of the abdominal segments. 

 When they contract, these muscles draw forward the terga 

 and thus straighten the abdomen. On the ventral side 



