272 CRUSTACEA. 



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 quarter of an inch 

 long, occasionally parasitic. The male surpasses all animals 

 in the brilliancy of its "phosphorescent" colour. 



Chondr acanthus. As in many other cases, the parasitic females 

 carry the pigmy males attached to their body. 



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



Lerncea, Penella, 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 understood 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 reproduc- 

 tive period. 



Arguhis, a divergent form temporarily parasitic on carp, &c. It 

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

 A protrusible spine projects in front of the blood sucking 

 mouth; the mandibles and first maxillae are adapted for 

 piercing ; the second maxilke or maxillipedes for adhesion. 

 There are four pairs of two-branched swimming appendages. 

 There are two large compound eyes. 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, which in adult life are fixed head down- 

 wards. The body is indistinctly segmented, and is enveloped 

 in a fold of skin, usually with calcareous plates. The anterior 

 antennae 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. The abdomen is rudi- 

 mentary. There is no heart. The sexes are usually com- 

 bined, out dimorphic unisexual forms also occur. The herma- 

 phrodite individuals occasionally carry pigmy or " comple- 

 mental " males. The spermatozoa are mobile, which is 

 unusual among Crustacea. 



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

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

 drawn out into a long flexible stalk, containing a cement gland, the 

 ovaries, &c., and involving in its formation the first pair of antennae and 

 the front lobe of the head. The second antennae are lost in larval life. 

 The mouth region bears a pair of small mandibles and two pairs of small 

 maxillae, the last pair united into a lower lip. The thorax has six pairs 

 of two-branched appendages, and from the end of the rudimentary 



