42 PHYLUM ARTHROPODA 



Class : Crustacea. Division : Entomostraca. Order : Copepoda 

 A FREE-SWIMMING COPEPOD (CYCLOPS) 



These minute animals are representatives of the division of 

 Crustacea called the Entomostraca. All the crustaceans here- 

 tofore studied belong to the higher group called Malacostraca. 

 Copepods are extremely common in both fresh and salt water. 

 They may be obtained in almost any permanent pool of water in 

 the woods or fields or from the surface water of the sea, often in 

 large quantities, and are easily kept in aquaria. The animals 

 should be studied alive if possible, and their swimming move- 

 ments observed. They propel themselves, not by means of the 

 feet, but by the long antennae. It is the rowing motion of these 

 organs which give the animals their jerky movements in the 

 water. Place them on a slide under a cover glass with a few 

 strands of cotton, and examine them under a microscope; in 

 order to prevent the cover glass from crushing them it is well to 

 support it with a Httle wax at two of the corners. If the pressure 

 of the cover glass does not suffice to keep them quiet, the with- 

 drawal of some of the water from under the cover glass with 

 blotting paper will probably accomplish this result, but care 

 must be taken not to let the water dry up. 



Observe the cylindrical body and the two pairs of long an- 

 tennae with their sense hairs ; also the long spines at the end of 

 the abdomen. Note the division of the body into abdomen and 

 cephalothorax, and also that the latter is not covered by a cara- 

 pace. If the animal is a female it may be carrying a pair of egg 

 sacs filled with eggs extending from the anterior end of the abdo- 

 men. Note the median eye, also the intestine and muscle fibers, 

 through the transparent body wall. 



The body is made up of fifteen somites, the head, thorax, and 

 abdomen each containing five. The head is relatively large, and 

 its somites are fused together ; they bear the cephalic appendages 

 common to all crustaceans. The first pair of antennae is longer 

 than the second ; in the male it is secondarily modified to form 

 clasping organs, by which the female is held during pairing. In 



