286 WILSON. [Vol. IX. 



two the ciliated larvae attach themselves to the wall of the 

 aquarium, and undergo a metamorphosis. 



Esperella, being of small size, is easily kept in aquaria, but 

 the larvae, as a rule, do not escape in great numbers. A con- 

 finement over night in a simple aquarium of good size, say 

 three gallons, through which no water is passing, will nearly 

 always result in the liberation of some larvae. Instead of 

 trying to aerate the water, it was found more convenient to 

 transfer the larvae with the help of a pipette to fresh dishes 

 of water. As most of them attach in a day or so, it is only 

 necessary to transfer a few times. Once attached to the wall 

 of the dish, the dish itself may be placed in a running 

 aquarium and the little sponges thus kept without further 

 trouble. I could not, however, succeed in getting them to 

 increase much in size, in spite of the aquarium facilities in the 

 Fish Commission laboratory. They seemed willing to live 

 indefinitely, but grow they would not — for lack of proper food, 

 I suppose. 



The young sponge after the metamorphosis is so thin 

 (scarcely more than an incrustation) that it cannot be scraped 

 off the dish without injury to it. I, therefore, coated my dishes 

 with a thin layer of paraffine. Collodion was also used. A 

 little piece of the paraffine, or collodion, could then be cut out 

 with the sponge sticking to it, and the whole thing placed in 

 the killing fluid, and subsequently kept in alcohol until ready 

 for use. In many cases the little sponge separates at once 

 from the parafifine in the killing fluid. I satisfied myself, by 

 comparison with larvae scraped from the dish, that the parafifine 

 or collodion did not affect the character of the tissues. To be 

 sure such larvae as remained stuck to the paraffine were only 

 kept in alcohol for a few weeks. If the action of the alcohol 

 on the parafline be kept up for months, I am not sure but the 

 effect on the tissues of the sponge is injurious. 



For fixing purposes I found very much the best fluid was 

 the mixture of acetic acid, alcohol, and osmic acid, recom- 

 mended by Zacharias (glacial acetic i part, absolute alcohol 

 4 parts, osmic acid few drops). I allowed this to act 10-20 

 mins. It is excellent for all stages of the development. 



