M. C. Mereschkowsky on the Hydroida. 247 
places the want of tentacles for the function of alimentation, 
as is very well shown in fig. 5, p. 64, of the ‘ History of British 
Hydroid Zoophytes’ by Mr. Hincks, as well as by thedescription 
which accompanies it*. In the Hydroids without metameres, in 
which the body is consequently very short and not flexible, 
the tentacles are always filiform, long, fine, and very supple ; 
their length sometimes even becomes very great, as, for ex- 
ample, in Monobrachium parasitum, mihit, which has only a 
single tentacle. 
This, then, is the explanation that, I think, may be given 
of the fact that the articulate type of the Hydroids is asso- 
ciated with short and capitate tentacles. 
This view is further supported by the fact that the capitate 
tentacles are exclusively met with in the order Athecata, or 
the Gymnoblastic Hydroids—that is to say, among the naked 
Hydroids,—and that, on the contrary, in the order Thecaphora, 
in which each hydranth is furnished with a hydrotheca or 
ealycle of chitine within which it can entirely withdraw itself, 
and which often may even be closed by a small operculum, 
we only find filiform tentacles. This is very easily explained, 
seeing that these Hydroids, which are very well defended from | 
all attacks of their enemies by the hydrothece, within which 
they can conceal themselves in case of danger, have no neces- 
sity for organs so well designed for defence as are the capitate 
tentacles. On the other hand, as the Thecaphora grow in 
very numerous colonies, the number of individuals sometimes . 
exceeding 1000, it is necessary for them to adapt themselves 
to the possibility of procuring food in sufficient quantity for 
so great a number of individuals living together. This adap- 
tation in the case in question consists in the number of long, 
fine, filiform tentacles appropriated to prehension with which 
each individual is provided becoming very great, greater than 
it usually is in the naked Hydroids. (There are generally 
not fewer than 16, most frequently 20, 22, 24, and sometimes 
30, 32, or more.) | 
Finally, I may mention another fact, which will serve in 
* In fact the flexibility of the body of such Hydroids as Cladonema 
radiatum, Stauridium productum, and others is excessively developed, and 
may very well compensate for the want of filiform tentacles in the func- 
tion of prehension of nourishment. But it is especially in Clavatella 
prolifera that the length, contractility, and flexibility of the body have 
attained their maximum; and it is, I think, by this cause that we may 
explain why it also has capitate tentacles, although not belonging to the 
articulate type. It furnishes the only example of capitate tentacles in a 
non-articulate type. 
+ See my paper, “ On a new Genus of Hydroids from the White Sea,” 
in this journal for September 1877, ser. 4, vol. xx. p. 220. 
