October, 1920.] 



The Canadian Field-Naturalist 



127 



Thousand islands, N. Y. ; Great Lakes; neighbour- 

 hoods of Montreal, Ottawa and Hull; various lo- 

 calities in middle and southern Ontario; a few 

 localities in the middle and southern parts of Yukon 

 Territory and the four western provinces, Manitoba, 

 Saskatchewan, Alberta and British Columbia. 



The freshwater-Crustacea known from the locali- 

 ties given above are in some cases both Malacostraca 

 and Entomostraca; in ether cases only one of these 

 two sub-classes; in again other cases only certain 

 orders belonging to one or the other of these sub- 

 classes have been collected; finally it is often only 

 certain families or genera which occur in these more 

 northern parts of the American continent. 



The freshwater Entomostraca are mostly circum- 

 polar in distribution, and are perhaps best treated 

 from this point of view (as will be seen from the 

 Canadian Arctic Expedition reports) ; and as the 

 available records of them have been published 

 rather fully in various countries, I do not intend to 

 include such, in this article, apart from the Phyllo- 

 poda (Branchiopoda). 



The freshwater Malacostraca occurring in Can- 

 ada and Alaska are, however, properly to be con- 

 sidered contmental forms, outrunners from their 

 much more numerous representatives in the United 

 States. It is interesting to recall in this connection, 

 that no Decapods or Isopods are known from the 

 arctic and subarctic regions of Canada and Alaska; 

 and that though the Amphipods are known to 

 occur all the way to the arctic coast of the main- 

 land (at least west of Hudson Bay), there are no 

 records of them hitherto from the islands compos- 

 ing the Canadian Arctic Archipelago, though they 

 are probably found at least on the more southern 

 islands. That no freshwater Malacostraca are 

 known from Greenland is also significant. The 

 details about this most interesting point (the dis- 

 tribution northward) will be given later in these 

 articles; suffice is it to say now that the Decapoda 

 going farthest north are certain species of cray 

 fishes; and of Isopds and Amphipods probably only 

 the three common forms, Asellus communis, Cam- 

 marus limnaeus and H^alella lfniclferbocf(eTi. 



Unfortunately we have practically no records of 

 freshwater Crustacea from the vast area outside 

 the localities given above for this continent, except 

 in so far as certain species (Cammarus limnaeus, 

 Hyalella }(nicl(erboclferi), which are distributed 

 over the whole of the mainland part of Canada and 

 Alaska or more southern species (Mancasellus tenax, 

 certain Amphipods and cray-fishes^, are concerned. 

 It is, therefore, most desirable that material be col- 

 lected in the following two areas; the whole sub- 

 arctic part of the continent from Alaska to the 

 Labrador Peninsula (Ungava), and the islands 

 composing the Canadian Arctic Archipelago. It is 



my hope, that future collectors in these regions will 

 pay far more attention to the freshwater-crustacea 

 than has been done heretofore. Each collection will 

 have considerable value, not only from a scientific 

 but also from an economic point of view, particular- 

 ly in a country like Canada where the freshwater 

 occupies at least fifteen per cent of its total area, 

 and the fish living therein which depend so largely 

 upon these Crustacea form one of our great national 

 assets. 



Amphipoda. 



The general appearance of these Crustacea, the 

 great majority of which are found in the sea, (about 

 two dozen species occurring in freshwater upon this 

 continent), may be supposed to be fairly well known 

 to the general public, who will have noticed them 

 in great numbers in the small pools around stones or 

 under sea-weed along beaches at low tide. They 

 are commonly called "shrimps," though this name 

 properly should only be used for certain "Decapod" 

 Crustacea (prawns, etc.) "Sea- weed-lice" or 

 "beach-fleas" are really better names and are 

 popularly used, for instance in the Scandinavian 

 countries. Ortmann (I.e.) gives "scuds" as the 

 popular English name for them. The main char- 

 acteristics of these Crustacea are an arched outline 

 and a compressed, many segmented body; the lack 

 of carapace and of stalked eyes; the feelers (an- 

 tennae) and legs are also considerably shorter than 

 in the true "shrimps." They swim mostly vertically 

 in the water by bendings of the body and rapid, 

 continuous movements of the paired "tail feet" 

 (pleopods), while the body-legs (peraeopods) help 

 in the balancing of the animal and the mouthparts 

 (maxillipeds) are kept ready for any food. When 

 they reach the water surface it will often be seen 

 that they seem unable to descend again, and swim 

 around in circles on their sides. The reason for this 

 is that the air gets in under the protruding parts 

 (pleura) of the body segments (somites), so that the 

 animals become lighter than the water. They feed 

 mainly upon decaying animal and vegetable matter, 

 and are therefore, especially the smaller forms, often 

 found among water plants, etc. It is well known 

 how quickly meat-bones, dead fishes, etc., lowered 

 into the sea or a lake are gnawed clean by these 

 Crustacea, so that only the skeleton-parts remain. 

 In size they range from a few centimetres to some 

 giant, marine, forms, several inches long. The fe- 

 males carry their many eggs in a sort of brood- 

 pouch on the underside of the body between the 

 legs, and the embryos go through their whole de- 

 velopment here, so that when they are "born" they 

 have practically the same appearance as their 

 parents, a rather unusual thing among the Crustacea. 

 Even the recently emerged young ones keep for a 



