The Copepods exhibit a great variety of structure, and their classification is attended with great difficulties. Claus[36] based his attempt at a natural classification on the character of the mouth and its appendages, dividing the free-living and semi-parasitic forms as Gnathostomata from the true parasites or Siphonostomata. This division, although convenient, breaks down in many places, and it is clear that the parasitic mode of life has been acquired more than once in the history of Copepod evolution, while the free-living groups do not constitute a natural assemblage. [Illustration: FIG. 25.—_Calanus hyperboreus_, × 30. _Abd^1_, First abdominal segment; _1st Ant_, _2nd Ant_, 1st and 2nd antennae; _Md_, mandible; _Mx^1_, _Mx^2_, 1st and 2nd maxillae; _Mxp_, maxillipede; _Th^1_, 1st thoracic appendage. (After Giesbrecht.) ] Giesbrecht has more recently[37] founded a classification of the free-living pelagic Copepods upon the segmentation of the body and certain secondary sexual characters, and he has hinted[38] that this scheme of classification applies to the semi-parasitic and parasitic forms. Although much detail remains to be worked out and the position of some families is doubtful, Giesbrecht’s scheme is the most satisfactory that has hitherto been suggested, and will be adopted in this chapter. The peculiarity in structure of the Argulidae, a small group of ectoparasites on fresh water fish, necessitates their separation from the rest of the Copepods (Eucopepoda) as a separate Branch, Branchiura. BRANCH I. EUCOPEPODA. =Sub-Order 1. Gymnoplea.= The division between the front and hind part of the body falls immediately in front of the genital openings and behind the fifth thoracic feet. The latter in the male are modified into an asymmetrical copulatory organ. TRIBE I. AMPHASCANDRIA. The first antennae of the male are symmetrical, with highly-developed sensory hairs. =Fam. Calanidae.=—The Calanidae are exclusively marine Crustacea, and form a common feature of the pelagic plankton in all parts of the world. Some species of the genus _Calanus_ often occur in vast shoals, making the sea appear blood-red, and they furnish a most important article of fish food. These swarms appear to consist chiefly of females, the males being taken rarely, and only at certain seasons of the year. Some of the Calanidae are animals of delicate and curious form, owing to the development of plumed iridescent hairs from various parts of their body, which may often exhibit a marked asymmetry, as in the species figured, _Calocalanus plumulosus_ (Fig. 26), from the Mediterranean. [Illustration: FIG. 26.—_Calocalanus plumulosus_, × 15. (After Giesbrecht.) ]