CLADOCERA. 



299 



Fig. 240. Actheres of the trout 



The highest members of the group of sucking Entomo- 

 straca are Caligus and A rgulus, in which the body ij seg- 

 mented, with antennae and free 

 mouth-parts and legs ; the latter 

 genus with compound eyes. Cali- 

 gus curtus Mtiller lives on the cod, 

 and Argulus aloscB Gould on the 

 alewife. 



Order 3. Branchiopoda. This 

 order includes such Crustacea as 

 in the higher forms breathe by 

 rather broad feet There is a con- 

 siderable range of 

 form from the 

 Ostracoda, repre- 

 sented by Cypris, 

 in which the feet 

 are much as in Cy- 

 clops, through Daphnia and Sida (Fig. 242) 

 which represent the Cladocera, up to the 

 Phyllopods. The suborder of Ostracoda 

 ( Cypris) are bivalved, the shell often thick. 

 They have two eyes, two pairs of antennae, 

 a pair of mandibles with a jointed feeler 

 (palpus) and a gill, and four pairs of feet, 

 the second pair often carrying a small gill. 

 The shells of certain species allied to Cypris 

 abound in the lowest Silurian strata. The 

 species live in fresh-water pools and in the 

 ocean at various depths. They undergo no 

 metamorphosis, the youngest stage being a 

 shelled Nauplius. 



The suborder Cladocera is represented by^ 

 fresh and salt-water species. The higher 

 forms are Sida and Daphnia. They are 

 called water-fleas from their jerky motions. 

 The eggs of Daphnia are borne about by 

 Pig 24i Peneiia of the females in so-called brood-cavities on 



the sword-fish, female. ^ back under ^ ghelL There are two 



sorts of eggs, i. e., the "summer" eggs, which are laid by 



