ARTHROPODA. 



241 



257. The Smaller Crustaceans. Less conspicuous and less highly 

 developed than these are many species, including a number of fresh- 

 water forms the "water-fleas." If the student makes any study 

 of stagnant waters he is sure to find these minute Crustacea darting 

 about the side of the vessels in the laboratory. The most common 

 type is Cyclops, named from the single eye in the middle line of the 

 head. Daphnia is another beautifully transparent form that is 



FIG. 87. Gammarus ornatus. From VerrilL 



Questions on the Figure. How does thi form compare with 

 other Crustacea studied in the differentiation of the segments, 

 in fusion of the segments, and in the differentiation of the append- 

 ages? 



'aprella geometrica. From Verrill. X 4. 



Question on the Figure. In comparison with other Crustacea, 

 what are the aberrant or peculiar features of this form? See also 

 figures in reference texts (e. g., Parker and Haswell's Zoology, Vol. 

 I, P- 546). 



very common. They both multiply very rapidly and keep close 

 up to the limits of the supply of the food on which they live. It 

 is said that one adult cyclops may in the course of a season produce 

 500 young, and since each of these becomes sexually mature in a 

 short period, it is estimated that the descendents of one cyclops, if 

 everything were favorable for their living, would amount to 4,500,- 

 000,000 individuals in a season. Of course this never happens in 

 16 



