A FREE-SWIMMING COPEPOD 43 



Cyclops, which is the commonest fresh-water genus of the Co- 

 pepoda, the first thoracic somite is fused with the head, leaving 

 only four free thoracic somites. The abdomen bears no append- 

 ages. In the female the first two abdominal somites may be 

 fused together. 



Exercise 1. Draw a large outline of the dorsal aspect of a copepod, 

 not putting in any appendages except the antennae. Represent 

 accurately the sense hairs on the antennae and the caudal bristles. 

 Number the thoracic and abdominal segments and carefully label 

 all the parts. 



Study the appendages. The thoracic appendages are bira- 

 mous. They do not bear gills, and the fifth pair is rudimentary. 

 The cephalic appendages consist of two pairs of antennae, one 

 pair of mandibles, and two pairs of maxillae. 



Exercise 2, Draw a side view of an animal showing the appendages 



in position. 

 Exercise 3. Draw an outline of a thoracic leg on a large scale, showing 



accurately all the joints and hairs and its biramous form. 



Compare the copepod with the young larva of the crab or the 

 lobster. Enumerate the points of structural similarity between 

 them. 



Internal Anatomy. This can be best studied in the live animal. 

 The alimentary tract is straight and of large diameter, and often 

 contains dark-colored fecal matter. The mouth has a ventral 

 position, as in other crustaceans, and the anus is dorsal. There 

 is no liver or other accessory glandular organ. The circulatory 

 system in Cyclops consists of the colorless blood fluid alone, 

 there being no heart. The blood is, however, kept in circulation 

 by the rhythmic contractions of the intestine. Other copepods 

 possess a dorsal heart. There are no special respiratory organs, 

 respiration being effected through the body wall. The excretory 

 system consists of a pair of coiled kidney tubes, called the shell 

 glands, which lie in the forward part of the cephalothorax. 



The reproductive system consists of median or paired organs in 

 the dorsal portion of the cephalothorax above the intestine. In 

 the female the ovaries are often conspicuous as a pair of large 



