u 4 COMPARATIVE ANATOMY 



underneath the cephalothorax, and it is the principal swimming 

 organ of the animal. As is well known, crayfishes, lobsters, 

 and all their tribe swim backwards by means of powerful strokes 

 of the abdomen which is alternately flexed by a rapid contrac- 

 tion of its ventral muscles and more slowly straightened out 

 by the dorsal muscles. The principle on which the joints of 

 an arthropodous animal are formed has already been explained 

 (p. 94), and is well exemplified in the abdomen of the crayfish. 

 Each segment consists of a ring of thick calcified cuticle, united 

 to its fellows before and behind by an intervening section of 

 soft cuticle. The upper surface of each ring is wide from front 

 to back, and evenly arched from side to side. This part is 

 known as the tergum. Its sloping sides are continued below 

 the level of the abdominal floor into a pair of broadly lanceolate 

 plates, the pleura. Ventrally the segment is completed by a 

 transverse skeletal bar, the sternum. The outer ends of the 

 sternal bar forms the inner sides of a pair of articular cavities 

 for the limbs, the outer sides of each cavity being formed by 

 a short skeletal piece extending to and continuous with the 

 pleuron. This piece is known as the epimeron. It should 

 be observed that the first abdominal somite has no pleura, and 

 is overlapped by the posterior edge of the carapace. In turn 

 it overlaps the second abdominal segment, and this and each 

 succeeding segment overlaps the one behind it. 



The telson is flattened dorso-ventrally, has no pleura, bears 

 no appendages, and is imperfectly divided by a transverse 

 joint into two pieces. 



In the cephalothorax the great dorsal carapace, when viewed 

 from above, has an oval contour truncated at both ends. From 

 the centre of the anterior end projects a beak-like prominence, 

 the rostrum. It has a broad base, ends in front in a sharp 

 spine, and has two principal and some smaller accessory lateral 

 spines. 



The carapace shows no trace of segmentation except that 

 the head region is marked off from the thorax behind by a 

 cervical groove running transversely across the carapace at 

 about the middle of its length and continued forwards and 

 downwards on each side towards the lower anterior edge of 

 the carapace. Behind the cervical groove a pair of longi- 

 tudinal branchio-cardiac grooves mark off a median cardiac 

 from the lateral branchial regions of the carapace. If one 



