M. C. Mereschkowsky on the Hydroida. 249 
must remain intact; for in proportion as the body became 
elongated, the tentacles lengthened likewise, so’ that they 
could always reach the mouth and convey food to it (fig. 7). 
But if the functions of the tentacles did not undergo any 
change, we need not expect them to change their form, except 
perhaps to become longer. This is what we remark in such 
forms as Cordylophora, Clava, &c., in which the lowest ten- 
tacles are not in the least shorter or less developed than the 
upper ones; on the contrary, they are sometimes a little longer; 
and in all cases they are all, without exception, longer than 
the body, owing to which they all have the faculty of assisting 
in the capture of prey, as has been very well described by 
Van Beneden in the case of Cordylophora*. 
I must still mention an articulate type, represented by the 
genera Tubularia, Acharadria, Corymorpha, Pennaria, &c., 
in which the superior metamere has 
the tentacles capitate, but much less Fig. 8. 
developed than those of the other me- 
tamere. All these forms are derived 
from a non-articulate form with tenta- 
cles so well developed, so long, and in 
such great quantity, that when the 
formation of the second metamere was 
induced by some cause, the tentacles 
belonging to it were perfectly useless 
to the organism, which caused them 
to become atrophied, and at the same 
time capitate—that is to say, adapted 
solely to the defence of the organism | ' 
(fig. 8). As the Medusa may be re- © Acharadria larync. 
garded as a hydranth reversed, and in 
which the tentacles (radial canals) are united together by a 
gelatinous substance (ectoderm or bell), it may be understood 
that the Medusz which have tentacles at the extremity of 
the manubrium belong to the same articulate type as Tubu- 
laria, Acharadria, &c., with two metameres, the inferior of 
which is more strongly marked than the upper. 
To complete this morphological chapter I propose to explain 
in a few words a point of view from which I regard all the 

quite recently, and that they are in process of forming capitate tentacles, 
or of lengthening them. Thus Mr. Allman remarks, “Some Hydroids 
with filiform tentacles show, like Clava squamata, a tendency to the ter- 
minal enlargement of the tentacles in certain states of contraction” 
(Allman, Mon. Gymnobl. Hydr. p. 245). 
* Van Beneden, ‘ Faune littorale de Belgique,’ Polypes. 
Ann. & Mag. N. Hist. Ser. 5. Vol. i. 17 
