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MANUAL OF ELEMENTARY ZOOLOGY 



body, known as the hind body or abdomen, is more com- 

 pletely segmented. At the end of the abdomen is a flat 

 piece known as the telson, on the under side of which 

 the anus opens. The telson bears no limbs, and is divided 

 by an imperfect transverse joint. The armour of each 

 segment of the abdomen consists of a broad back-piece or 

 tergum and a narrow belly-plate or sternum, with a pair of 

 V-shaped prolongations, known as the. pleura, joining them 



at the sides. There are no 

 pleura on the first abdominal 

 segment. The tergum, ster- 

 num, and pleura of each seg- 

 ment form a continuous ring. 

 The limbs are jointed to the 

 hinder side of the sternum near 

 its outer ends, and the part 

 of the sternum between each 

 limb and the adjoining pleuron 

 is sometimes called an epi- 

 meron. The terga overlap one 

 another from before back- 

 wards and slide over one 

 another as the abdomen is 

 straightened and bent, the 

 armour of each segment being 

 joined to that of the next by 

 thin cuticle, which allows of 

 movement. In the cepha- 

 lothorax the terga are fused 

 to form a shield or carapace. 

 This is prolonged in front into 

 a beak-like rostrum and is 

 crossed by a furrow, which is called the cervical groove be- 

 cause it is supposed to mark the separation of two regions 

 known as the head and thorax. At each side of the body 

 a fold of the carapace overhangs as a lean-to roof, the gill 

 cover or branchiostegite, which encloses between itself and the 

 side of the body a chamber in which the gills lie. Behind the 

 cervical groove a branchiocardiac groove on each side marks 

 off the branchiostegite from a median cardiac region which 

 roofs the thorax. The cuticle of the inside of the branch- 



FlG. 153. — View of a crayfish 

 from above. — After Huxley. 



Note cervical groove, cardiac region of 

 carapace, rostrum, and tail fan. 



