A COPEPOD 53 
CRUSTACEA 
A FREE-SWIMMING COPEPOD (Cyclops) 
These minute animals are representatives of the division of 
Crustacea called the Entomostraca. All of the crustaceans 
heretofore studied belong to the higher group called Malacos- 
traca. 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. Place several 
on a slide under a cover-glass and examine them under a micro- 
scope. If the pressure of the cover-glass does not suffice to 
keep them quiet, the withdrawal of some of the water from 
under the cover-glass with blotting-paper will probably accom- 
plish this result. Also stain and mount a number of copepods 
in balsam or glycerine. Observe the cylindrical body and the 
two pairs of long antenne 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 acarapace. If the animal be a female it may be 
carrying a pair of egg-sacs filled with eggs extending from the 
anterior end of the abdomen. 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 append- 
ages common to all crustaceans. The first pair of antenne is 
longer than the second; in the male it is secondarily modified 
