CLASSIFICATION OF CRUSTACEA. 271 



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



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



(Pre-genital).i The i6th in the female carries an egg sac or brood 

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

 Abdominal f 17-68. Fifty-two abdominal feet, to which there corres- 

 ( Post-genital). \ pond 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 i6th append- 

 ages. 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. Professor Ray Lankester called this 

 cephalic ganglion an "archi-cerebrum," to emphasise its pre- 

 oral position and its distinctness from the posterior ganglia. 

 Subsequent research has shown, however, that in Apus, as in 

 other Crustaceans, the cephalic ganglion is a " syn-cerebrum, 

 i.e., it is composed of pre-oral ganglia fused with post-oral ganglia 

 which have been shunted forwards. 



(b) Cladocera. Small laterally compressed "water fleas," with 

 few and somewhat indistinct segments. The shell is usually 

 bivalved. The head often projects freely. The second pair 

 of 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 fertilisa- 

 tion, 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. 

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



