4'2U ARXniiOPODA. 



segment is two-forked, forming the 'furca.' While the abdomen 

 lacks ajopendages, the thorax bears typical biramous apj^endages, 

 consisting of a two-jointed basioi)odite, the basal joint Ijeing fre- 

 quently united with its fellow of the pair for common motion (fig. 

 410, /). Exopodite and endopodite, usually three-jointed, are 

 fringed with bristles. Usually the fifth pair of thoracic ap)pend- 

 ages are not so well developed, and in some cases are represented 

 by two bunches of bristles. 



The two pairs of antennge are frequently similar in size (whence the old 

 name Cijclops quadricornis). The first pair are always uniserial and in the 

 males may be hooked near the base for clasping; the second are some- 

 times biramous (fig. 410, 11). The mandible (fig. 410, III, F) is instruc- 

 tive, since a study of several species shows that it is derived from a sohiz- 

 opodal condition and that the first basal joint alone is used for chewing, 

 the rest being reduced to a palpus of varying development. Both basal 

 joints of the maxilke (fig. 410, lY) can be used in eating. Two maxilli- 

 peds (formerly regarded as the separated branches of an appendage) mark 

 tlie termination of the head (fig. 422, .-;); both are hooked for holding the 

 food. 



The internal anatomy is simple. There is no liver, and tlie 

 straight intestine (fig. 422) runs without marked changes in size 

 to the anus between the branches of the furca. The visual organ 

 is tlie unpaired nauplius eye (which has given the name to one 

 genus, Cydop&). It lies directly on the brain. Tlie ventral chain 

 has its ganglia irregularly distributed. Gills are always absent, as 

 are usually the heart and blood-vessels. Only in a few parasitic 

 forms are there tubes which have been interpreted as jjarts of a vas- 

 cular system; in some free forms there is a short saccular slowly 

 pulsating heart. The gonads are unpaired in both sexes, but the 

 sexual ducts, which open at the base of the abdomen, are paired. 

 The females possess a receptaculum seminis distinct from the ovi- 

 ducts, to which the male attaches the spermatophores packed with 

 sperm (fig. 422, sp). As the eggs leave the oviduct they are fer- 

 tilized by the sperm issuing from the spermatophores, and num- 

 bers are enclosed in a gelatinous substance, thus producing bundles 

 of eggs, the so-called egg-sacs, attached to the abdomen, by which 

 one can easily recognize the females (fig. 7). A nauplius hatches 

 from the egg, and by budding segments and appendages at the 

 hinder end, and by a change of the nauplius appendages into 

 antennse and mandibles, passes through a • cyclops-stage ' into the 

 adult. 



The Copepoda have clearly descended from some phyllopod- 

 like form. The poorly developed ventral chain, tlie loss, partial 



