203 



d.ah.a. 



ext.tn. 



REGENERATION AND AUTOTOMY 



The power of regeneration, though it is less in the crayfish 

 than in earthworms and much less than in Hydra, is still con- 

 siderable. A whole limb which is injured can be grown again. 

 The injured leg is first cast off by a spasmodic contraction of 

 some of its muscles which 

 causes it to break through at 

 the basipodite, the internal 

 cavity — which, as we shall 

 see, is a blood space — being 

 here crossed by a partition 

 which leaves only a small 

 opening, through which the 

 nerves and blood vessels pass. 

 When the limb is cast off this 

 opening is quickly closed by a 

 blood clot, after which the 

 cuticle grows across the 

 wound. Beneath the scar the 

 new limb is formed as a bud 

 and gradually takes shape. 

 At the next moult it becomes 

 free, though it is still small, 

 and it increases in size at 

 each moult, until a normal 



limb has been provided. This power of casting off limbs is known 

 as autotomy. It is sometimes used as a means of escape from 

 enemies which have seized one of the limbs, but this is not so 

 common in the crayfish as in some animals that are related to it. 



v.n 



v.ah.a. 



Fig. 136. — A semi-diagrammatic drawing 

 of a transverse section of the abdomen 

 of the crayfish. 



bp. 



Basipodite ; cp., coxopodite ; d.ab.a., dorsal 

 abdominal artery ; en., endopodite ; ex., exo- 

 podite ; ext.m., extensor muscles ; ji.m., flexor 

 muscles ; h.g., hind-gut ; pi., pleuron ; pr., 

 protopodite ; tg., tergum ; st., sternum ; v.ab.a., 

 ventral abdominal artery ; v.n.c, ventral nerv'e 

 cord. 



PERIVISCERAL CAVITY AND ALIMENTARY SYSTEM 



The body of the crayfish contains a spacious perivisceral 

 cavity, in which the internal organs lie. This is not a coelom, but 

 an enlarged portion of the haemocoele (p. 189), and communicates 

 with the blood vessels. The alimentary canal fills the greater part 

 of this cavity. The mouth is an elongated opening below the head 

 between the mandibles. It has in front a wide upper lip or labrum, 

 and behind it is a pair of lobes (paragnatha) known together as 

 the lower lip or metastome. A short, wide gullet leads upwards 



