78 CRUSTACEA—COPEPODA CHAP. III 
hosts from salt to fresh water, or the reverse. America appears 
to be the home of the Argulidae.' 
The structure of an Argulid is exhibited in Fig. 48. In 
front of the siphon, within which the styliform mandibles and 
first maxillae work, there is a poison-spine (sp); the appendages 
which correspond to the second maxillae (mx) are modified into 
sucking discs, but in the genus Dolops they terminate in normal 
claws. The next pair of appendages, usually spoken of as maxilli- 
pedes (mp), are clasping organs, and behind follow four pairs of 
thoracic swimming feet (1-4). The body is foliaceous, and they 
always apply themselves to their hosts with the long axis pointing 
forwards and parallel to that of the host, while on various parts 
of the under surface of the body are spines pointing backward 
which prevent the parasite being brushed off by the passage of 
the host through the water. These animals, alone among the 
Copepoda, possess compound eyes. 
A short sketch has now been given of the variations in 
Copepod organisation, but we cannot leave the subject without 
pointing out the rich field which still remains for the 
morphologist, especially in determining the true relationships 
of the parasitic families. 
1 C, B. Wilson, Proc, U.S. Nat. Museum, xxv., 1902, p. 635, 
