THE CRAYFISH 



249 



another, more powerful, set of muscles connects the sterna 

 with one another and with the endophragmal skeleton. 

 These, when they contract, draw closer the sterna and thus 

 bend the abdomen. By this movement, spreading at the 

 same time its tail fan, the crayfish carries out the sudden 

 backward jumps by which it escapes from its enemies. Its 

 gentle forward movements are carried out by the walking 

 legs, aided by a paddling of the abdominal limbs. The 

 legs of the first three 



pairs Dull and those d.ab.a. 



of the' last pair push, ^^^C^' 7 "" 



and their movements . jig2SkiUim$^ ./ { 9" 



are carried out in 

 such a way that the 

 animal is always 

 standing upon six 

 legs while two — 

 which are on opposite 

 sides and of different 

 pairs — are in motion. 

 The body of the 

 crayfish 

 c o ntains 

 a spaci- 

 ous peri- 

 visceral cavity, in 

 which the internal 

 organs lie. This is 

 not a ccelom, but 

 an enlarged portion 

 of the hasmoccele 

 (p. 230), and com- 

 municates 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 is 

 bordered in front by a wide upper lip or labrum, and behind 

 it stands a pair of lobes known together as the lower lip or 

 metastoma. A short, wide gullet leads upwards into the 

 large proventriculus, often called the "stomach." This 

 consists of two chambers, a large forepart or mill-chamber, 

 often known as the "cardiac division of the stomach," and 



Perivisceral 

 Cavity and 

 Alimentary 

 System. 



u.ri.e, 



Fig. 158. — A semi-diagrammatic drawing of 

 a transverse section of the abdomen of 

 the crayfish. 



/'/<., Basipodite ; cp., coxopodite ; d.ab.a., dorsal 

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

 podite ; ext.m., extensor muscles ', fl.m., flexor 

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

 protopodite ; tg., tergum ; St. , sternum; v.ah.a.., 

 ventral abdominal artery ; v.n.c. ventral nerve 

 cord. 



