844 FRESH-WATER BIOLOGY 



27 (28) Carapace coalesced dorsally with not more than three of the thoracic 

 somites. Thoracic Mmbs with natatory exopodites, first 

 pair modified as maxillipeds. Pleopods more or less re- 

 duced and greatly different in the two sexes. Eggs carried 

 in a brood pouch at the base of the thoracic legs. 



Order Mysidacea. 



This order forms part of the old division Schizopoda. The Mysidacea live chiefly in salt 

 water. The system of this group needs a thorough revision, and no satisfactory division into 

 larger groups (families) has been published. 



Only species in North America. . . . Mysis relicta Loven 1862. 



Fig. 1310. Mysis relicta Loven. X 2. (AEter Smith.) 



Very few Mysidacea are known from the fresh water, and the present is identical with a 

 species Hving in lakes in northern Europe (Ireland, Scandinavia, Russia). It is found, in 

 North America, under similar conditions, in the lakes Superior and Michigan, down to a 

 considerable depth (150 fathoms). 



In Europe, this form has been much discussed, and, as the name indicates, was supposed to 

 point to a former connection between the sea and the lakes in which it lives. It was believed 

 that these lakes were cut off from the sea, and became fresh-water lakes, but retained part of the 

 original marine fauna adapted to the fresh-water conditions: these animals were called " marine 

 relics," and Mysis relicta was taken for one of the most prominent examples of this kind. How- 

 ever, this theory has been greatly shaken recently, and, as far as it concerns the North American 

 stock of Mysis relicta, there is no reason to assume that it is a marine relic, but we are to regard 

 it as an immigrant into the Great Lakes in glacial times (as Lysianassa). 



28 (27) Carapace coalesced dorsaUy with all of the thoracic somites. Tho- 

 racic limbs rarely with exopodites, the first three pairs modi- 

 fied as maxiUipeds. Pleopods not much reduced, and not 

 very different in the two sexes, except the anterior ones. 

 Eggs carried under the abdomen, attached to the pleopods. 



Order Decapoda . . 29 



29 (34) Body and rostrum compresssed. Pleura of second abdominal 

 somite overlapping those in front. First two pairs of perae- 

 opods chelate. Anterior pleopods of the male not trans- 

 formed into copulating organs . . 30 



