ENTOMOSTRACA. 303 



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 antennae. 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. Its 

 appendages are : antennules, antennae, mandibles, first 

 maxillae, second maxillae, four pairs of flattened biramous 

 thoracic legs united across the middle with those of the 

 opposite side, another rudimentary pair, and probably the , 

 genital valve. Cetochilus^ Calanus, free and abundant in 

 the sea. In Chondracanthus , as in many other cases, the 

 parasitic females carry the pigmy males attached to their 

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

 the carp-lice (Argulus} the mouth is a sucker with sharp 

 stilets and the second maxillae form adhesive discs. 

 Lernaa, Penella^ etc. The adult females are parasitic, and 

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



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 rudimentary. 

 There is no heart. The sexes are usually combined, but 

 dimorphic unisexual forms also occur. The hermaphrodite 

 individuals occasionally* carry pigmy or "complemental" 

 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, etc. , 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 rudi- 

 mentary abdomen a long penis projects. At the base of this lies the 

 anus. Around the body there is a fold of skin, and from this arise five 

 calcareous plates, an unpaired dorsal carma, two scuta right and left 



