266 CRUSTACEA. 



[6. First thoracic foot (leg-like). 

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

 (Pregenital). 1 The 16th in the female carries an egg-sac or brood- 

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

 Abdominal / 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. 



{b) 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 antennae 



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 maxillae. 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. Within 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. 



Older 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). 



Older 3. Copepoda. — Elongated Crustaceans, usually with distinct seg- 

 ments. Theie 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. Many are ecto-parasitic, especially on fishes ("fish-lice"), 

 and are often very degenerate. The free living Copepods form an 

 important part of the food-supply of fishes. 



Cyclops, free and exceedingly prolific in fresh water. Cetochilus, 

 free and abundant in the sea. Sapphirina, a broad flat marine 

 form, about a quartet of an inch long, occasionally parasitic. 

 The male is remarkable for its brilliant "phosphorescent" 



