KEY TO THE ENTOMOSTRACA OF COLORADO 



267 



The mandibles are the most constant pair and, as in other Crustacea, 

 are the chief jaws. The two pairs of maxillae are, on the whole, not 

 greatly developed, though in all groups one or both pairs serve to 

 some extent as accessory jaws. The appendages of the trunk are 

 of special interest. In all the Branchiopoda they are typically flat- 

 tened, and their primitive function is that of swimming organs. 

 They have the special peculiarity that the base of each one is modified 

 on the medial side as a gnathobase, or chewing base, so that even the 

 appendages far from the mouth assist in capturing, in carrying to 

 the mouth, and in chewing the food. In the more primitive groups 

 this type is closely adhered to, but in the more specialized types there 

 are varying degrees of departure. The primitive forms have a large 

 number of similar, foliaceous trunk appendages, whereas in the more 

 advanced groups they are fewer and more diversi- 

 fied. In the Copepods they have lost the primi- 

 tive foHaceous form and are definitely rounded and 

 biramous. The Ostracods show the greatest reduc- 

 tion, there being but two pairs in the trunk region. 

 For a tabulated comparison of appendages of the 

 different groups see Table I. 



The common means of collecting Entomostraca 

 is with a net of fine mushn or, better, of bolting 

 cloth (No. 10 is fine enough). A good form is a 

 conical net with an opening of about 5 inches and 

 a length of 15 inches. The material which collects 

 in the bottom of the net can be washed out by 

 everting the net into a wide-mouthed bottle or 

 other vessel. A better plan is to have in the 

 small end of the net a little funnel which may be 

 stopped with a cork. Such a net may be towed 

 behind a boat by means of a cord; or if the funnel 

 is weighted with about 2 oz. of lead, it may be thrown out from the 

 shore 50 to 75 feet when no boat is available. 



The material may be studied alive or in fresh condition with best 

 results, but when necessary it may be preserved. Although 70 per 



Fig. I. — Net used in 

 making collections. 



