438 NOTES. 



s\rim at the bottom of the aquarium, on account of its relative lightness, 

 and that, by degrees, it acquires the power of doing so, it has been 

 inferred that this acquired power is dependent on the impregnation of 

 its tissues by the salt water. This, however, has not been proved, and 

 there is another way by which the fish may be able to alter its specific 

 gravity by the reabsorption of the air contained in its air-bladder 

 and constantly renewed in it. In highly aerated fresh water so much 

 air is deposited in this air-bladder and in the vessels generally, that the 

 fish is rendered lighter than the water, and cannot go to the bottom 

 even in fresh water. In my aquaria I have seen Sticklebacks, Bleak, 

 and Axolotl perish from the superabundance of air in their tissues in 

 consequence of the constant addition of highly aerated water. On the 

 other hand, a diminution of the air contained in the air-bladder might, 

 of course, easily occur, and thus the specific gravity of the animal would 

 be raised while the whole volume of its body remained the same. 



Note 60, page 154. As Beudant's small work is not easily accessible, 

 I shall present the reader with an epitome of his tables on the next 

 page. 



It must, however, be observed that in this inquiry no regard is paid 

 to temperature. Now, since, under variations of temperature, the 

 respiratory requirements of the cold-blooded animals are extremely 

 different, we may be allowed to assume that a due regard to this circum- 

 stance would have led to somewhat different results from the same 

 experiments. 



Note 61, page 154. At four miles east of Kiel there is a fossil oyster- 

 bed. ' Thousands of years after the oyster-bed of Waterneversdorf had 

 become dry land, oysters lived in such numbers on the coasts of the 

 Danish islands that they were used for food by man of the " stone age " 

 in this region.' Mobius is inclined to attribute the failure of the oyster 

 in the Baltic to its low degree of saltness, combined with the long 

 duration of a low winter temperature, and the absence of any regular 

 movement of the sea by the tide ; he assigns the same reasons for the 

 absence of the lobster, the large crab, Platycarciniis pa gurus, and the 

 edible sea-urchin, Echinus esculentus. 



Note 62, page 155. Prof. Verrill, of Yale Coll., U.S., one of the most 

 accurate students of the American Crustacea, in a conversation I had 

 with him on this subject, disputed the accuracy of this estimate of 

 Schmankewitsch. He said : ' The only characters which can be relied 

 upon for distinguishing the genera Branchipus and Artemia are the 

 male prehensile organs, and these have been entirely overlooked by 

 Schmankewitsch.' I have neither time nor materials at my disposal for 

 a close investigation of this point, and I will only observe that in his 

 latest work he does take the prehensile antennae of the male into con- 

 sideration. 



