in EUCOPEPODA GYMNOPLEA AMPHASCANDRIA 57 



evolution, while the free-living groups do not constitute a natural 

 assemblage. 



Giesbrecht has more recently l founded a classification of the 

 free-living pelagic Copepods upon the segmentation of the 

 body and certain secondary sexual characters, and he has hinted 2 

 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 



1 Fauna aiul Flora G. v. Neapel, Monograph 19, 1892. 

 2 Ibid. Monograph 25, 1899. 



