136 INVERTEBRATE ANIMALS. 



Cladocera may be taken the " branched-homed water-flea " 

 [Daphnia pulex, Fig. 56, 5), thousands of whicli may be 

 captm'ed in any pond in summer. In this pretty little species 

 the whole body is enclosed in a bivalve shell, which is so 

 transparent that the whole organization of the animal is clearly 

 visible through it. The head is distinct, and carries a single 



Tia 56. — Frosh water Entomostraca. a Cijpris i/ri^- striata; 5 . 

 c Cyclops guaOjricomis. 



eye. The greater antennse are branched. The males are 

 smaller than the females, and much fewer in number ; and it 

 appears to be a well-established fact that the female, when 

 once fertilized by the male, can not only lay eggs for the rest 

 of her life, but can transmit the power of producing fertile 

 ova to her young for several generations. Of the Copepoda 

 one of the commonest is the Cyclops (Fig. 56, c), in which 

 the cephalo-thorax is covered by a shield, and there is a well- 

 developed abdomen. The female carries on either side a kind 

 of pouch or ovisac, in which the eggs remain till they are 

 hatched. The httle Ostracoda (Fig. 66, a) are all minute 

 Crustaceans, which occur in both fresh and salt water. They 

 are distinguished by the fact that the body is entirely enclosed 

 in a shell, which is made up of two lateral halves or valves. 

 The valves of the shell are united . by a membrane along the 

 back, but can be opened below, so as to allow of the protrusion 

 of the feet. 



Oedbe Cieeipedia. — The last order of Crustacea which re- 

 quires mention is that of the Cirripedia (Lat. cirrus, a curl ; 

 and pes, foot), comprising the so-called barnacles and acorn- 



