152 C. F. R0U3SELET ON BRACHIONUS SERIOUS, 



Mr. Gosse's figures, which really represent slight varieties of 

 B. urceolaris. 



The lorica of both species is smooth, glassy transparent, 

 rounded dorsally, and of about the same size. Anatomically, 

 also, the two species can hardly be separated, and the foot- 

 opening in both is rounded in the ventral plate and square 

 dorsally. 



The shape of the frontal teeth of the lorica therefore is the 

 main distinguishing mark ; but in addition B. rubens has a 

 unique habit, mentioned by Ehrenberg as having been observed 

 already by Schaffer in 1755, of attaching itself by its toes to 

 water-fleas, sometimes covering the whole surface of the shell of 

 this crustacean, and thus causing poor Daphnia considerable 

 impediment and discomfort. The Brachioni readily detach them- 

 selves when so disposed, swim about in the water for a while and fix 

 themselves again on the back of the next water-flea they come 

 across. They appear quite at home there, lay numerous eggs, 

 and breed freely. In ponds where Daphnia is not numerous 

 every one of them has its load of B. rubens when present. 

 If the Daphnia be killed, all the Brachioni speedily abandon the 

 dead host. One can imagine a Daphnids delight when, in 

 moulting, which may occur once in three or four days, it can slip 

 out of its "old clothes" and suddenly escape from its unbidden 

 guests. It is hardly a case of symbiosis, for the advantages 

 are all on one side. 



An interesting biological problem is presented by the fact that 

 B. rubens has learned to make systematic use of Daphnia pulex 

 as a means of easy locomotion, whilst the closely allied species 

 B. urceolaris, living in identical surroundings, has not yet 

 acquired this useful habit. It seems clear that there must exist 

 a corresponding difference in the intellect or in the quality of the 

 brain of these two species which has enabled the one to discover 

 and make use of a means of locomotion which the other has not 

 yet thought of, and seems incapable even of imitating. The 



