50 LEPTODORA HYALINA. 



wherever they occur they are found in large numbers, They appear to 

 avoid strong light ; thus, in clear sunshine none are found at the surface, 

 though they may abound at a depth of a few feet ; at night they occur 

 quite at the surface, but bright moonlight is sufficient to drive them 

 down again. The best time for capturing them is in gloomy weather, 

 towards evening, or on dark nights. Weismann suggests, however, that 

 this shunning of the light may be only apparent after all, and due to the 

 fact that Cyclops, which, according to him, is the chief food of Leptodora, 

 has the same habit, so that Leptodora may be merely following its prey 

 about, instead of being itself directly influenced by light. Still the fact 

 is one of some interest, and one a knowledge of which may greatly 

 facilitate the capture of this interesting Entomostracon. 



Swimming is effected entirely by means of the posterior antennae, 

 and is comparatively slow. It is only in extremest necessity, when stuck 

 fast, that the abdomen is used for progression, while the feet appear 

 to be never used for this purpose. 



Cladocera may be divided into a littoral group and a deep-water 

 group, to the latter of which Leptodora belongs. It is only found in 

 water clear from vegetation, and away from the shore. When kept in 

 captivity, algae and particles of dirt soon attach themselves to the 

 swimming arms or antennae, and the animals rarely survive so long as a 

 fortnight. They are not uncommonly attacked by a fungus, which growa 

 inwards through the integiimcnt and gradually kills them. 



Weismauu describes Leptodora as habitually lying quietly stretched 

 out in the water, like Corethra, and waiting till its prey comes within 

 reach of its foot- jaws ; he considers that the terminal dilatation of the 

 alimentary canal partially counterbalances the thorax and head, and so 

 aids in the maintenance of the horizontal position. 



The relations of Leptodora to the other Cladocera are discussed by 

 Weismann at considerable length. He points out how the long segmented 

 abdomen of Leptodora is correlated with the rudimentary condition of its 

 carapace, and shows how in such forms as Bythotrephes and Polyphemus 

 we have a gradual increase in the size of the carapace, accompanied by a 

 gradual diminution in the length of the abdomen and fusion of its 

 different segments ; while finally in Daphnia the carapace has grown 

 backwards and downwards, so as to completely enclose the whole animal 

 —a chauge accompanied by complete fusion of thorax and abdomen, in 

 which no indication of the constituent segments is visible in the adult. 

 Tbe reduction of the body and fusion of the segments is carried to a still 

 higher point in the Ostracoda, such as Cypris, in which also the bivalve 

 shell reaches its highest development. 



The absence of branchial lamellae on the legs in Leptodora is also to 

 be correlated with the rudimentary condition of the carapace ; these 

 lamellae, which attain their greatest development in Lynceidas and 

 Daphniadae, in which also the carapace is most fully developed, probably 

 serve to maintain a current of water through the shell, rather than to 

 act themselves as direct respiratory organs, and they are therefore need- 

 less when the shell is absent or rudimentary. 



