
559 THE AMERICAN NATURALIST. | [Vor. XXXVI. 
on Aurelia, as well as with the recent work of Hein on the 
same form. 
The larval history seems to vary greatly as to matters of 
growth, transformation, etc. A few specimens attached them- 
selves to the bottom of the aquaria in from six to ten days and 
promptly assumed the typical scyphistoma stage, while others 
were swimming planula at the end of as many weeks, showing 
no tendency toward transformation. 
Encystment. — Both McMurrich and Hyde have recorded an 
encysting phase in the life of the planula preceding metamor- 
phism. Hyde noted, however, that in one series of experi- 
ments this did not take place. My own observations have 
probably sufficed to reconcile these discrepancies by showing 
that it is incidental rather than essential, — a mere adaptation 
to changed conditions. 
That encystment may and does occur under certain circum- 
stances is not in the least doubtful, if not indeed a more or 
less common phenomenon in development under artificial con- 
ditions. A very considerable proportion of my own specimens 
passed through such a stage, which varies from a few days to 
many, and is, indeed, often a permanent and fatal one ; but, on 
the other hand, it is not less certain that many passed through 
the metamorphoses without the least sign of encystment, while 
in others there was the aspect of incipient encystment common 
in many hydroid planule, namely, the secretion of a delicate 
perisarc-like sheath over the base of the planula in the process 
of attachment. There seems little doubt, therefore, that in 
these variable aspects we have the clue to the entire matter, 
so far as these larva are concerned, and may consider encyst- 
ment, as above intimated, to be an adaptation against unfa- 
vorable conditions, and therefore comparable with the same 
phenomenon among Protozoa and other organisms. May it 
not hint the probable phyletic origin of the perisarcal structure 
“so largely characteristic of one class of ccelenterates, to which 
the facts under consideration may show more or less definite 
reversion ? 
T) he Scyphistoma. — The young scyphistoma is at first of 
vasiform shape, the base small and adherent by the delicate 

