PHYLUM ARTHROPOD A 



213 



arthropods. In the free-Hving forms, the body is usually 

 elongated and distinctly segmented. The appendages are char- 

 acteristically biramous, though some have become so modified 

 that they have lost their biramous nature. Six pairs of append- 

 ages are borne on the head and four or five on the anterior region 

 of the trunk, while the posterior region of the trunk lacks append- 

 ages. The last abdominal segment is forked. 



The impaired nauplius eye is characteristic of copepods and 

 prompted the application of the name Cyclops to one genus. 



In many instances, the eggs when discharged from the female 

 are surrounded by a gelatinous substance and remain attached 

 to the body as prominent egg sacs. Conspicuous egg sacs are 

 highly characteristic of the female Cyclops. Larvae hatching 

 from the eggs are in the nauplius stage. 



Copepods occur in such abundance and are important food 

 items for such numbers of different animals that they have 

 considerable economic impor- 

 tance. Many fishes feed almost 

 exclusively upon small crusta- 

 ceans of which the copepods are 

 the most numerous. Even some 

 whales subsist largely upon 

 copepod diet. 



There are a number of fami- 

 lies displaying an extensive array 

 of bizarre body forms. In many 

 instances, all of the appendages 

 excepting those modified for 

 attachment to the host have 

 degenerated. 



Parasitic Copepods. — In the foregoing, only the free-living 

 copepods have been considered. There are immense numbers 

 of copepods which dwell as parasites upon other animals. Fishes 

 are especially prominent hosts of those parasites, which are 

 popularly known as fish lice. Both the body and gills are sub- 

 ject to attack. Some of these parasitic copepods e.g., the genus 

 Argulus (Fig. 98) of the order Branchiura have a pair of com- 

 pound eyes, fully developed swimming feet, and a modification of 

 the first maxillipeds to form a pair of sucking discs for securing 

 attachment to the host. These argulids are not permanent 

 parasites, for especially at the breeding season they leave the 



Fig. 98. — Dorsal view of a female 

 parasitic copepod, Argulus versicolor. 

 {After Wilson). 



