86 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The 6th abdominal segment (20 in Figs. 25 and 26) differs 

 from the preceding, mainly in the great size of the swimmerets. 

 The protopodite is broad, thick, and undivided, while two broad 

 oval plates fringed with setae represent endopodite and expodite, 

 the latter being two- jointed. The last abdominal segment or 

 telson (21) possesses neither pleura nor appendages. It is divided 

 into two by an imperfect transverse joint, and upon its little- 

 calcified sternal surface the anus (a) opens as a longitudinal slit 

 with thickened edges. The telson and large 6th pair of swim- 

 merets make up together the tail-fin. The 1st and 2nd abdominal 

 segments mainly differ in the character of their appendages, but 

 in addition to this the former is small, possesses no pleura, and 

 hinges on to the cephalothorax in a somewhat complicated manner. 

 In the female, the 2nd pair of abdominal appendages are swim- 

 merets, the 1st pair, when present, rudimentary swimmerets. 

 The appendages of these segments are modified in the male into 

 copulatory organs (Fig. 26, 15 and 16; Fig. 27, K and L). 



In the cephalothorax (Figs. 25 and 26) the tergal and pleural 

 regions are covered by a continuous carapace, produced in front 

 into a sharp spine, the rostrum (r). On the upper surface of the 

 carapace is a transverse groove, which slopes downwards and 

 forwards to its front edge. This cervical groove (c.gr) marks the 

 boundary between the head and thorax. 



(2) The thorax is composed of eight fused segments, the last 

 of which is slightly movable. In many of the lower Crustacea 

 the thoracic segments are freely movable upon one another like 

 the abdominal segments of the crayfish, and particular interest 

 attaches to the genus Nebalia, which is in many ways transitional 

 between the lower and higher forms. This is a small shrimp- 

 like animal with a thorax composed of eight segments and covered 

 by a large fold growing back from the head. The part of the 

 crayfish's carapace behind the cervical groove probably corresponds 

 to such a fold, which in this case, however, has fused closely with 

 the thorax. The tergal region of this fold is rather narrow, and 

 its lateral boundaries are indicated by a branchio -cardiac groove ( x ) 

 running back from the cervical groove. The pleural region of 

 the fold on each side is a large curved plate, the branchiostegUe (br), 

 which bends outwards and downwards to the bases of the thoracic 

 appendages and covers a large gill-chamber. It is united with the 

 head along the cervical groove. The epimera are very large and 



