532 CRUSTACEA ORDERS COPE POD A AND CIRRTPEDIA. 



Daphniidce, or water-fleas are small Crustacea which abound in fresh water. 

 They are of a compressed oval form, and are partly covered by a thin, trans- 

 parent bivalve shell. The first pair of antennae is small, but the second very 

 large, bifid at the extremity, and set with long hairs, which serve as the 

 principal organs of locomotion. The eyes are fused into one large one, near 

 which is often another small one. They have from four to six pairs of 

 swimming legs. They are very favourite objects with microscopists, being 

 abundant, and admitting of easy observation. In the Apod id fe the body is 

 covered with a shield-shaped carapace, and there are two contiguous eyes, 

 and from 30 to 40 pairs of gill-feet. Apus caiicriformis, Schiiff., is a brownish 

 yellow species, about two inches long, which is found in ponds. The large, 

 oval carapace covers all but the last few segments of the body, which ends in 

 a flat double tail, on each side of which a long bristly appendage is visible. 

 This family is of interest as having formed the subject of Mr. Bernard's 

 memoir, already alluded to, in which he connects the Annelida with the 

 Trilobites through the Apodidw. The Branchipodidw have no shell, but a 

 strongly-segmented body, stalked eyes, and a long tapering tail, ending in a 

 flat pair of flattened appendages set with bristles. The most interesting 

 species of this family is the brine shrimp, Artemia salina (Linn.), which is of 

 a pink colour, and about half an inch long. It is more shrimp-like in appear- 

 ance than any other species of Entomostraca which we have mentioned, and 

 is only found in brine pits, appearing to prefer the most concentrated brine. 



ORDER II. OSTRACODA. 



In the Ostracoda the body is not segmented, and is entirely covered by a 

 bean-shaped bivalve shell. There are four antennae set with bristles, and 

 one or both pairs are used for swimming and grasping. There are five pairs 

 of other appendages, of which the first, second, often the third, and some- 

 times even the fourth, serve the purpose of jaws, and only the last pair or 

 pairs perform the office of legs. 



ORDER III. COPEPODA. 



In this order there are ten swimming-legs at most, generally bifid, and set 

 with hairs and bristles. One or two sessile eyes 

 are present, and the mouth is furnished with leaf- 

 like gills. The large eye possessed by some of 

 the species led to their being placed in a genus 

 Cyclops by Miiller. They commence life in the 

 Nauplius-foTm, to which the full-grown animal 

 still exhibits a strong resemblance. Some species are 

 free, and others parasitic ; as, for instance, those 

 belonging to the family Notodelphyidce, which live 

 within the branchial cavity of Ascidia. A few 

 species like these are marine, but the majority 

 of the Cydopidce live in fresh water, one species, 

 Fig. 5.- CYCLOPS QUABRI- Canthocamptvs alpestris (Vogt) being found in 

 CORNIS. Magnified. the glacier waters of the Aar in Switzerland, at an 



elevation of 8,500 feet. The family Argulidce in- 

 cludes some species which are parasitic on the carp, and which are known as 

 "carp lice." 



