50 A CYCLOPS. 



STRUCTURAL DETAILS. The body of a Cyclops is somewhat pear- 

 shaped, broad and rounded in front and tapering behind. Note the 

 division into segments, characteristic of the Arthropods. The upper 

 surface can be distinguished from the lower by its more convex form and 

 by the absence of the jointed limbs, which are attached to the under 

 side. Distinguish the following: 



a) The cephalothorax fused head and one segment of the thorax 



comprising the anterior third of the body, and covered dorsally 

 by the carapace. If material is at hand compare this carapace 

 with a similar structure in the crayfish and in Daphnia, or in one 

 of the small so-called bivalve crustaceans often seen swimming 

 about in aquaria or ponds. 



b) The four free thoracic segments following the cephalothorax. 



How do they compare in width and in length ? 



c) The abdomen, comprising the rest of the body. How many seg- 



ments has the abdomen, if we regard the foremost one as made up 

 of two fused together ? 



d) As your specimen is probably a female, there may be a pair of egg- 



sacs attached near the front of the abdomen. The eggs were 

 extruded into these through a small opening on each side at the 

 point of attachment of the sacs. The openings mark the line of 

 fusion of the first two abdominal segments. 



e) Projecting from the front of the cephalothorax is a pair of long, 



tapering jointed appendages, the antennules. Do you notice the 

 animal making any use of these? If you happen to find a male 

 specimen you will observe that the antennules are curiously modi- 

 fied, so that one portion closes up on the other like the blade of a 

 jack-knife. 



f) Behind the antennules is a smaller pair of appendages, the antennae. 



These will be partly, or perhaps entirely hidden beneath the body. 

 By proper manipulation you can make the ends at least to show. 



g) The single median eye near the front of the carapace. Examine 



this carefully with a lens to see if there is any evidence of the 

 fusion of two eyes. In what way is this eye-spot associated with 

 the name of the animal ? 



