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nucleus. It soon acquires a more elongated form, exhibits a mani- 
fest contractility of its walls, becomes clothed with vibratile cilia, 
and, finally, by the rupture of the confining structures, escapes as 
a free locomotive embryo into the surrounding water. 
I have not as yet met with any instance of the occurrence of the 
extra-capsular sac in a male gonophore ; and it would thus seem to 
constitute a kind of marsupial appendage, into which the ova are 
introduced from the interior of the gonophore, in order to undergo a 
further development previously to their final liberation as free larvae. 
It occurs, with slight modifications, in several other species, and 
ought to have a place in the descriptive terminology of the group. 
I therefore, in allusion to its position on the summit of the gono- 
phore, propose for it the name of Acrocyst. 
We must be careful not to confound the acrocyst with an extras 
capsular, medusa-like sporosac, which in certain species shows itself 
in a similar situation, but with which it is in no respect homologous, 
— the acrocyst being in an entirely different morphological category 
from that of the sporosacs. 
Sertularia pumila , Linn. 
The gonophores in this species are compressed, urn-shaped bodies, 
generally containing a single sporosac. 
From the enlarged opercular summit of the blastostyle there 
extends into the interior of the gonophore a variable number of ccecal 
tubes. Some of these are in the form of short, simple, cylindrical 
processes, while others extend to a greater distance, become more or 
less branched, and may be traced in contact with the inner surface 
of the walls of the capsule to within a short distance of the attached 
end of the gonophore. Brown corpuscles, similar to those in the 
interior of the blastostyle and ccenosarc, may occasionally be seen in 
active motion within these coeca. 
The sporosac originates as a lateral bud from the blastostyle, and 
contains a large, simple spadix, surrounded by th§ ova or spermatozoa. 
The generative elements, however, are not confined to the sporo- 
sac. In the blastostyle itself numerous ova may be detected ; but 
these are always smaller, and evidently less mature, than those in 
the sporosac. They are produced in the walls of the blastostyle, 
apparently between the endoderm and ectoderm, and are very minute 
towards the summit of the blastostyle, but gradually increase in 
