286 PH \ 'L UAf AR 7 'HKOPODA . 



I 3. Mandible. 

 ( )ral. - 4. Maxilla. 



( 5. Maxillipede. 



?6. First thoracic fout (leg-like). 



Thoracic | 7-16. Other ten thoracic feet (swimmers). 



(Pregenital). j The i6th in the female carries an egg-sac or brood - 



V. chamber. There are eleven thoracic rings on the body. 

 Abdominal J 17-68. Fifty-two abdominal feet, to which there corre- 

 ( Post-genital). \^ spond only seventeen rings on the body. 



The large dorsal shield is not attached to the segments behind the 

 one bearing the maxillipedes. Many of the thin limbs doubtless 

 function as gills. The genital apertures are on the sixteenth 

 appendages. The anus is on the last segment of the body. 



There is a pair of ventral ganglia to each pair of limbs ; the ventral 

 nerve-cords are widely apart ; and the cephalic ganglion is 

 remarkably isolated. 



(/>} Cladocera. Small laterally compressed "water-fleas," with few 

 and somewhat indistinct segments. The shell is usually bivalved, 

 and the head often projects freely from it. The second antenna- 

 are large, two-branched, swimming appendages, and there are 

 4-6 pairs of other swimming organs. The heart is a little sac 

 with one pair of openings. An excretory organ (the shell or 

 maxillary gland) opens in the region of the second maxilla:. It 

 is the Entomostracan equivalent of the antennary green gland 

 of Malacostraca. The males are usually smaller and much rarer 

 than the females. The latter have a brood-chamber between 

 the shell and the back, ^^ 7 ithin this many broods are hatched 

 throughout the summer. Periodic parthenogenesis (of the 

 "summer ova") is very common. "Winter eggs," which 

 require fertilisation, are set adrift in a part of the shell modified 

 to form a protective cradle or ephippium. 



Daphnia, Moina, Sida, Polyphemus, Leptodora, and many other 

 " water-fleas," are extraordinarily abundant in fresh water, and 

 form part of the food of many fishes. A few occur in brackish 

 and salt water. 



Order 2. Ostracoda. Small Crustaceans, usually laterally compressed, 

 with an indistinctly segmented or unsegmented body, rudimentary 

 abdomen, and bivalve shell. There are only seven pairs of 

 appendages. 



Examples. Cypris (fresh water), Cypridina (marine). 



Order 3. Copepoda. Elongated Crustaceans, usually with distinct seg- 

 ments. There is no dorsal shell. There are five pairs of biramose 

 thoracic appendages, but the last may be rudimentary or absent. 

 The abdomen is without limbs, and of its five segments the first 

 two are sometimes united. The females carry the eggs in external 

 ovisacs. Most Copepods move very actively in the water, jerking 

 themselves rapidly by means of their thoracic legs, or swim more 

 gently by means of their second antenna 3 . Many are ecto-parasitic, 

 especially on fishes ("fish-lice"'), and are often very degenerate. 



