88 



The Canadian Field-Naturalist, 



[Vol. XXXV. 



THE LARGER FRESHWATER-CRUSTACEA FROM CANADA 



AND ALASKA. 



By Frits Johansen. 



(Continued from Vol. XXXV, p. 47.) 



Ill C. CLAM-SHRIMPS. 



This suborder (Conchostraca) is easily 

 distinguished from the two preceding ones 

 hy the presence of a double shell enclosing 

 the animal completely (except when swim- 

 ming) and attached to it dorsall.v, so that 

 it can be opened and closed like a clam 

 (see p. 45). Conforming with this the 

 bodj^ is much compressed, but shows a dis- 

 tinct head-part, sometimes beak-shaped 

 (rostrum) and protruding from the "shell. 

 The eyes are sessile and more like those of 

 the "water-fleas" (Cladocera) than those 

 of the fairy-shrimps. In other features 

 also they resemble the Cladocera, and may 

 be considered to stand half way between 

 this group of animals and the Phyllopods. 

 Thus the first pair of antennae are minute, 

 but the second pair developed into long 

 and powerful swimming organs. Of mouth- 

 organs both mandibles and two pairs of 

 maxillae are present; and the short, clum- 

 sy body is supplied with from 1 to 2 dozen 

 pairs of foliaceous legs, subdivided both 

 for respiratory and swimming purposes; 

 they also support the female egg-mass 

 dorsally, while the first pair serve with the 

 male as clasping organs ("hand" and 

 "claw"). The posterior body segments 

 have each a pair of spines often present 

 also on the compressed, large, terminal end 

 (telson), which ends in a pair of filaments 

 (cercopods). In size these animals are 

 from a few millimeters to about 2 centi- 

 meters long, the Estheriidae being the lar- 

 gest forms. 



Differeiit from the Cladocera (where 

 the young ones when emerging are very 

 much like the adults) the clam-shrimps 

 hatch as nauplii not unlike those of the 

 fairy-shrimps, though the first pair of 

 antennae are not yet out and the labrum 

 much larger. But the other characters 

 (enormous second pair of antennae and 

 mandibular palps, single, median eye, pear- 

 shaped body with little differentiation of 

 the segments, etc.), are the same as with 

 the other Phyllopods; also (as is the case 



with the Notostraca-nauplii) the carapace 

 is not yet developed. Gradually it assu- 

 mes the shape of the adults by the out- 

 growth of the shell (carapace) on both 

 sides behind the mouthparts, the shortening 

 of the body, reduction of the mandibular- 

 palps, and development of the paired eyes, 

 foliaceous legs, etc., and sexual characters. 

 It should be remembered, however, that of 

 many clam-shrimps the larval stages are 

 very imperfectly known or not known at 

 all. 



Their food consists of still smaller in- 

 vertebrates (Entomostraca, Protozoa, etc.), 

 which they secure by busily swimming 

 around in circles in the water ; as a matter 

 of fact when observing them they seem 

 never to be at rest. Their occurrence is 

 extremely puzzling ; ^ a water-hole a few 

 feet wide may be teeming with them, whe- 

 ther it is situated in the woods, in a grassy 

 swamp, or on open, dry plains'; on the 

 other hand they may be entirely absent 

 from other pools, ponds or lakes in the 

 same place. As is the case with the fairy- 

 shrimps they seem mainly to occur (around 

 Ottawa) in pools or canals left by the 

 overflow of rivers, ^ and are found from 

 April to the time the pools dry up (July). 

 In the summer the females carry their eggs 

 around with them ; they are deposited then 

 before the pool they are found in dries up 

 or freezes to the bottom. The females 

 have also (Packard, Sars) been noticed to 

 carry their eggs in the spring (see below). ' 



It is interesting that so far there are no 

 records of them from the high Arctic (ex- 

 cept Siberia), though, as mentioned, both 

 fairy-shrimps, tadpole-shrimps and water- 

 fleas have their typical representatives up 

 there. I did not find them on the Can- 

 adian Arctic Expedition, nor are thev 

 known from Greenland. My identifications 

 of the Conchostraca from the new Can- 



1 Some years they do not show up at all. 

 -' The genus Limnetis seems to prefer pools 

 with much vegetation. 



3 Or holes filled with water by heavy rains. 



