782 FRESH-WATER BIOLOGY 



1 19 (i 16) Furca with three setae. . Canthocamptus northumbricus Brady 1880. 

 This is probably, next to minutus, the most widely distributed species in North America. 



1 20 (109) Spines of anal plate bifid. . . Canthocamptus minutus Claus 1863. 



This is the most common species and is found everywhere in the north- 

 ern continents. 



FIG. 1235. The last segment and furcae of male Canthocamptus minutus. 

 (After Schmeil.) 



SEPHONOSTOMATA 



THE parasitic Copepoda pass all or a part of their lives as para- 

 sites upon fish and other animals. They are exceedingly numer- 

 ous in both salt and fresh water, and very interesting because of 

 the strange forms which many of them assume, forms which 

 would appear to be in no way related to the structure of a copepod. 

 Many of them would be taken for worms. Some bore into the 

 tissues of their hosts, others dwell in the gills, and still others in 

 the nasal cavities. One species is very abundant on the sheeps- 

 head of the Central States. 



The appendages are profoundly modified to adapt them to their 

 parasitic existence. The swimming feet are more or less rudimen- 

 tary. The appendages about the mouth are modified into sucking 

 or prehensile organs. The antennae are similarly modified. In 

 some the second antennae are armed at the end with hooks to 

 enable the animal to retain its hold on its host. In some that are 

 semiparasitic, the appendages from the opposite sides are joined 

 together in a sucker. Sometimes the segmentation of the body 

 disappears entirely. The appendages in some are reduced to mere 

 protuberances, or may be like roots penetrating the body of the 

 host. 



And yet all these forms are free-swimming in their early stages. 

 When hatched from the egg they have the typical nauplius form 



