TYPE CRUSTACEA. 391 



ternal shell is sloughed and serves as a protecting case, but 

 more usually, as in DapTinia, Moina. and others, a saddle- 

 shaped thickening, the epMppium, appears on the dorsal wall 

 of the brood-pouch at the time of the passage of the winter 

 egg into it, and this thickening is thrown off with the egg and 

 forms a protective covering for it. 



3. Order Ostracoda. 



The Ostracoda resemble the Cladocera in the segmentation 

 of the body being but slightly marked and in possessing a 

 bivalved shell provided with an adductor muscle. The shell, 

 however, encloses the head as well as the thoracic and ab- 

 dominal regions, and furthermore but two thoracic limbs 

 exist. 



The antennules and antennae are both uniramous append- 

 ages and serve for creeping, though the former are also pro- 

 vided with olfactory hairs. The mandible consists of a tooth- 

 bearing plate and a strong jointed palp which in some forms 

 also functions as a creeping limb, and behind it are two well- 

 developed maxillae. The first of these is distinguished by the 

 development of the jaw portion and the reduction of the palp, 

 and in Cypris and Cythere bears a lai-ge plate with numerous 

 marginal setae which is usually termed a branchial lobe. The 

 second maxilla, on the other hand, shows considerable modi- 

 fication in different genera. In Cypridina (T?ig. 176, Mx') it is 

 jawlike and bears a large branchial lobe (wanting on the first 

 pair), and in Cypris is adapted for the same function, but bears 

 in addition to the rudimentary branchial lobe a short two- 

 jointed palp, which in Halocypris becomes enlarged to form a 

 three- or four-jointed limb, while finally' in Cythere the append- 

 age is practically a walking limb, its jaw function not being 

 developed. The first thoracic appendage is an elongated 

 many-jointed limb except in Cypridina (Fig. 176, t'), where it 

 possesses a jaw function, and the second is also limblike. lu 

 Halocypris this latter appendage is, however, rudimentary, and 

 in Cypris and Cypridina (Fig. 176, T') it is dorsally directed 

 and serves for cleansing the inner surface of the shell from 

 foreign bodies, in the latter genus arising some distance up 



