III EUCOPEPODA—GYMNOPLEA—AMPHASCANDRIA 57 
evolution, while the free-living groups do not constitute a natural 
assemblage. 
Giesbrecht has more recently’ founded a classification of the 
free-living pelagic Copepods upon the segmentation of the 
body and certain secondary sexual characters, and he has hinted * 
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 and Flora G. v. Neapel, Monograph 19, 1892. 
2 Ibid. Monograph 25, 1899. 
