CRUSTACEA OF ALABAMA. 33 



ORDER CLADOCERA. 



This large order of exclusively fresh-water shelled Crustacea is quite well represent- 

 ed in Alabama, in spite of the absence of large lakes which afford the most suitable lo- 

 cations. In a small pool at Opelika, for example, a very large variety was noted. The 

 Cladocera fauna of the south is characterized by the presence of several interesting in- 

 termediate forms not found at the north. 



The order is composed of animals of minute size and, in spite of great variation in 

 form, very obvious relationship. No doubt ever exists as to the right of a species to a 

 place within the order. So abundant are these creatures that no standing body of fresh- 

 water is long without some representatives. They may be recognized by the following 

 characters. The body is covered, with the exception of the very large laterally com- 

 pressed head, by a thin diaphanous shell, often beautifully striate or figured. The head 

 is provided with a large compound eye, a small occular fleck which is but a persistent 

 larval organ, a pair of small sensory antennules and a pair of large biramose antennae, 

 one ramus of which is three- the other four-jointed in most species. The mandibles are 

 nearly cylindrical and are attached to the shell at the juncture of head and body. A pair 

 of minute maxillae follow and then from four to six pairs of variously formed feet. The 

 feet are lamellate and serve a respiratory function. The body terminates in a flat 

 toothed body, the post-abdomen, which may be regarded as the result of the combina- 

 tion of the two anal stylets of other Crustacea. Sometimes this coalescence has involved 

 with it the alimentary canal, at other times not, so that the anal opening varies greatly in 

 position. 



The internal organization is very simple and is easily studied because of the trans- 

 parency of the body. Without expatiating on anatomical and systematic details for 

 which the reader is referred to special monographs, we pass to a description of the new 

 or remarkable species encountered in Alabama. 



The normal or calyptomerous cladocera are divided into two sections of which the 

 Ctenopoda are regarded the highest. The species of this group have six pairs of feet 

 and an elongate graceful body. The family Sididae, represented by the ubiquitous Sida 

 crystalline/, is the only one represented in Alabama. 



Two species of Daphnella and one of Sida are widely distributed, and near Mobile 

 occur the curious intermediate form described beyond. 



Genus Pseudosida. Herrick. 



In appearance very similar to Sida, but rather more slender than either species, 

 though very similar to Sida elongata ; antennules of the female with a long flagellum like 

 that of the male in Sida ; antennae with the outer (longer) ramus two-jointed, with nu- 

 merous setae, while the inner ramus is three-jointed and has but few setae, terminal seg- 

 ment short ; post-abdomen armed with groups of short spines. The antennae are like 



