84 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



function. Since the body is covered by a firm calcified cuticle, 

 segmentation and jointed appendages are a mechanical necessity 

 to an animal of such active habits. 



(1) The abdomen, as the least modified part, affords a con- 

 venient point of departure for study. It is made up of seven 

 segments, movably joined together, the first six of which bear 

 appendages. The abdominal region, like all the rest of the body, 

 is covered by a firm chitinous exoskeleton which is largely calcified, 

 but parts of it remain soft and joints are 'thus formed. The 3rd, 

 4th, and 5th abdominal segments are most typical, and similar 

 in the two sexes. The exoskeleton of each of these segments 

 consists of a calcified ring connected by uncalcified parts with 

 the segments in front and behind. The broad, strongly convex 

 dorsal and lateral part of the segment is the tergum (t). The 

 front part of this is overlapped by the preceding tergum, and its 

 hinder part overlaps the one behind it. Between the two extends 

 an uncalcified intertergal membrane, which is folded when the 

 tail is straight. The sides of the terga of adjacent segments 

 are united by peg and socket joints, allowing of upward and 

 downward movements. The tergum passes down on each side 

 into a small pointed projection (pi), the pleuron (larger in the 

 female), which is V-shaped in transverse section, being made up 

 of an outer and an inner limb. On the under side of the segment 

 is a slender transverse bar, the sternum (st), and broad uncalcified 

 intervals, the intersternal membranes, separate adjacent sterna. The 

 sternum passes on each side into a very short but broader epimeron 

 (epri), the junction between the two being marked by the attach- 

 ment of the appendage. The foregoing parts form a perfectly 

 continuous ring with no sharp lines of demarcation. 



The appendages are swimmerets (Figs. 26 and 27, M), and each 

 of them is a small elongated A' sna P e d limb, consisting of a prox- 

 imal* stalk, the protopodite (pr), and two distal* branches, an 

 internal endopodite and external exopodite (ex). 



The protopodite is made up of a very short proximal and a 

 larger distal joint. The endopodite and exopodite both possess a 

 relatively long proximal joint, and a rather longer distal part 

 imperfectly divided into rings. All parts of the swimmeret are 

 more or less beset with stiff bristles or setce. 



* The proximal and distal ends of an appendage, &c., are the ends respec- 

 tively further from and nearer to its free end. 



