114 A Synopsis of the Recent British Ostracoda. 



Ostracoda tlie segments of the body are almost entirely 

 obsolete,, the body itself being soft, and entirely covered by 

 two usually hard and calcareous, but sometimes thin and horny, 

 valves. 



The family Cypridse, which includes almost all the fresh- 

 water, and a few of the marine Ostracoda, may be taken as 

 the basis of our description. The various organs will be better 

 understood by reference to the plates, where they are figured 

 in detail. 



The first limb, or upper antenna (Fig. 1, &),here consists of 

 a tapering seven-jointed curved appendage, which bears at its 

 apex a dense bundle of long plumose setee. The second, or 

 lower antenna (Fig. 1, c), is usually stouter, four-jointed, 

 strongly bent or "geniculate," clawed at the extremity, and 

 adapted for walking, though in many cases it is also pro- 

 vided with a lash of seta?, which adapts it likewise for 

 swimming. The mandible (Fig. 1, d) consists of an elongated 

 triangular body, the base of which is directed downwards, and 

 is divided into numerous teeth. From one side springs a 

 four-jointed palp (Fig. 1, /), whose basal joint bears one of 

 the branchial organs (Fig. 1, e). Of the two pairs of jaws, 

 the first (g) is the larger, and is divided into four segments ; 

 to it is attached a large branchial plate (h), the principal 

 breathing organ of the animal. The second pair (i) is small, 

 and has a non-articulate palp, which, in the male, is often 

 modified into a prehensile organ. In some genera this pair of 

 jaws bears also a small branchial plate. There are two pairs 

 of feet, the first (j) clawed at the extremity, and adapted for 

 walking ; the last (h) slender, flexuous, and always tucked up 

 within the shell. The post- abdomen (m) consists of two 

 flattened elongated rami, which are very movable, strongly 

 clawed at the extremity, and lie side by side, mostly within 

 the shell. There are sometimes two eyes (a), but these are 

 mostly confluent. The ovaries (o) lie round the body of the 

 animal, directly beneath the shell. The copulative organs of 

 the male are of very curious and complex structure, and have 

 mostly attached to them a mucous gland (Fig. 4), consisting 

 of a double central cylinder, and several whorls of radiating 

 filaments. The shell is thin and fragile, and mostly somewhat 

 reniform or ovate in shape, devoid of sculpture, except some- 

 times a fine impressed punctation j occasionally densely hispid 

 or even spinous. 



We shall now briefly point out the characters in which the 

 other families chiefly differ from the Cypridaa. 



Cythekim*:. — This family includes by far the greater number 

 of the marine Ostracoda, and some few species are inhabitants 

 of fresh water. Both pairs of antennas (Fig. 2, l> } c) are en- 



