CLASSIFICATION OF CRUSTACEA. 245 



Order 2. Ostracoda. Small Crustaceans, with an indistinctly seg- 

 mented 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 distinctsegments. 

 There is no shell. There are four (or five ?) pairs of two-branched 

 thoracic appendages, and a five-jointed abdomen. The females 

 carry theeggs in externalovisacs. Manyareecto-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. Cetochiliis 



free and abundant in the sea. 

 Sapphirina, a broad fiat marine form about quarter of an inch 

 long, occasionally parasitic. The male surpasses all ani- 

 mals in the brilliancy of its ' ' phosphorescent " colour. 

 Chondracanthus. Here and in many others, the parasitic females 



carry the pigmy males attached to their body. 

 Caligus, a very common genus of " fish-lice." 

 Lemma, PeneUa, etc. The adult females are parasitic, and 

 almost worm-like. The males and the young are free. 

 That the males are often free and not degenerate, while 

 their mates are parasitic and retrogressive, may be under- 

 stood by considering ( i ) the greater vigour and activity 

 associated with maleness, (2) the fact that parasitism 

 affords safety and abundance of nutrition to the females 

 during the reproductive period. 

 Argulus, a divergent form temporarily parasitic on carp, etc. 

 It has a shield-like cephalothorax and a small cleft abdomen. 

 A protrusible spine projects in front of the blood-sucking 

 mouth. There are four pairs of two-branched swimming 

 appendages. There are two large compound eyes. The 

 testes lie on each side of the abdomen. The female has 

 no ovisacs ; the eggs are laid on foreign objects. 



Order 4. Cirripedia. Barnacles and acorn-shells, and some allied 

 degenerate parasites. 



Marine Crustaceans, whichin adultlifearefixedhead downwards. 

 The body is indistinctly segmented, and is enveloped in a fold 

 of skin, usually with calcareous plates. The anterior antennje 

 are involved in the attachment, the posterior pair are 

 rudimentary. The oral appendages are small, and in part 

 atrophied. In most there are six (or less frequently four) 

 pairs of two-branched thoracic feet, which sweep food 

 particles into the depressed mouth. There is no heart. The 

 sexes are usually combined, but dimorphic unisexual forms 

 also occur. The hermaphrodite individuals occasionally 

 carry pigmy or " complemental " males. 



Lepas, the ship-barnacle, is as an adult attached to floating logs and to 

 ship-bottoms. The anterior end by which the animal fixes itself is 



