390 INVERTEBRATE MORFHOLOGT. 



and Polyphemus, the branchial lobes being at the same time 

 rudimentary or entirely wanting. The abdomen, which is 

 composed of four segments, possesses on its dorsal surface 

 elevations for the closure behind of the brood-chamber, and 

 on its terminal segment setae are usually developed ; it does 

 not, however, bear any appendages. 



The heart is an oval structure situated in the thoracic re- 

 gion and possesses but a single pair of ostia. The lateral 

 eyes are in all cases fused to form a double eye situated in 

 the median line of the head and capable of movement within 

 a socket by means of muscles which are attached to it. 



The majority of the Cladocera are fresh- water forms, 

 though some, such as Evadne, are marine. The ova undergo 

 development in a brood-chamber formed by the space in- 

 cluded between the shell-valves and the dorsal surface of the 

 abdomen, and in Evadne and Polyphemus, as already stated, the 

 entire shell, which is somewhat reduced in size, is adapted to 

 serve as walls for the chamber. 



As in the Branchiopoda, collections of Cladocera, es- 

 pecially if made during the spring or summer, will show an 

 enormous preponderance of females, and several generations 

 may be reared without a single male making its appearance. 

 The eggs, which have a thin egg-membrane and little yolk, 

 develop parthenogenetically and produce females, and this 

 method of reproduction will continue so long as the condi- 

 tions, such as temperature and food, remain satisfactory; 

 hence the eggs of this kind are generally known as " summer 

 eggs." Towards autumn, however, or whenever the condi- 

 tions tend to become unfavorable, males, distinguishable by 

 their smaller size, the absence of a brood-pouch, and their 

 more highly-developed sense-organs, as well as by the de- 

 velopment of hooked setse on the anterior appendages which 

 serve as clasping organs, make their appearance, and at the 

 same time the females begin to deposit ova much larger in 

 size than the summer eggs and containing a considerable 

 amount of yolk. These " winter eggs " develop apparently 

 only after fertilization. In Polyphemus they possess a thick 

 shell, but in other forms special arrangements occur to render 

 them resistent to cold, drying, etc. In some forms the ma- 



