ENTOMOSTRACA. 



23 



Turning to the technical characters, we have to note that the anterior segments 

 of the body are covered with a carapax ; the feet are few in number, not exceeding 

 five pairs ; a single eye is present, and the segments 

 of the body are well marked. Like most of the En- 

 tomostraca, the Copepoda hatch from the egg in the 

 nauplius stage, from which the growth to the adult is 

 gradual, no startling metamorphoses being introduced 

 with the moults. 



Of all the Copepoda none is more widely dis- 

 tributed or better known than Cyclops, the type 

 of the family CYCLOPID^E. It is found in ponds 

 and slow streams all over the world, and is the 

 most common crustacean inhabitant of our drink- 

 ing-water. 



It has a body of a long-oval or pear shape, with 

 a large anterior shield, to which succeed four large seg- 

 ments, followed in turn by four smaller joints, forming 

 a tail, which is terminated by two projections armed 

 with bristles. 



In front of the animal appears the small com- 

 pound eve, the primitive crustacean eye, which is 

 found in a rudimentary form in so many of the higher 

 members of the Crustacea, where it has been func- 

 tionally replaced by a larger and more perfect com- 

 pound eye. The reten- 

 tion of this relatively 



imperfect sense-organ as the only organ of sight is one 

 among several features which mark this as one of the 

 most primitive groups of Crustacea. 



The large, straight intestine, usually filled with food 

 and kept in constant motion, occupies most of the body 

 cavity. By its churning action it keeps the blood in 

 motion, and thus fills the place of a heart, an organ 

 wanting in this genus, though present in other Copepoda. 

 At the proper season the large ovary may also be seen, 

 and often numerous fat globules, sometimes aggregated 

 into large masses. 



Of the appendages the antennules are functionally 

 the most important. They are long, stout, many-jointed, 

 and serve as most vigorous organs of locomotion. The 



fj *j 



antennaa are much smaller, and the mouth parts, man- 

 dibles, and pairs of maxilla are fitted for biting. There 

 are four pairs of large, two-branched legs, which are used 



a 



FIG. 28. Cyclops fluviatilis. 



in swimming. 



FIG. 29. Cyclops canthocarpoides. 

 a. Antennulse. b. Eyes. c. Oviduct. 

 d. Carapax. n. Ovisac. 



These Copepoda have various methods of locomo- 

 tion. Most frequently they ply their antennules like a 

 pair of oars, and assist their progress by strokes of the 

 tail, using it as a sort of sculling oar. The exact style of 



