. Fresh-water biology. Freshwater biology. 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 pu
. Fresh-water biology. Freshwater biology. 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 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
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