286 WILSON. [VoL. 1X. 
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 aérate 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 toit. 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 paraffine in the killing fluid. I satisfied myself, by 
comparison with larvae scraped from the dish, that the paraffine 
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 paraffine 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. 
