242 M. C. Mereschkowsky on the Hydroida. 
multiplication by means of incomplete transverse division 
(unvollstdindige (Juertheilung)—that is to say, the appearance 
of one or several new systems of secondary axes, which 
are all, at first, in accordance with the law of heredity, equal 
among themselves and to the first axial system from which 
they have proceeded. But instead of separating from each 
other and entering upon a free and independent life, as we 
see in a very analogous process of gemmation in the Scy- 
phistoma of the Discophorous Meduse (fig. 3)*, each system 
of secondary axes remains connected with a small community 
and leads a social life (fig. 4). 
In order to demonstrate that this view is correct, and that 
the articulate type is nothing more than the product of an 
incomplete transverse division, we may consider the singular 
anomaly presented by Clytia potertum, Ag. (fig. 2). There 
is no doubt that this form is produced by increase of growth, 
which for its part produces a a transverse division analogous to 
that which takes place in the Scyphistoma, but with the dif- 
ference that here the superior articulation does not separate 
from the colony, because the division is incomplete. But if 
we imagine the Hydroid deprived of its calycle we shall have 
fundamentally the same picture that is presented by the digram- 
matic figure of an articulate Hydroid in fig. 1. 
Let us now consider fig. 5, which represents, after Mr. 
Hinckst, the interesting Hydroid Vorticlava proteus. Owing 
to its great contractility it can take on different forms; and 
one of them (fig. 5), in which the superior metamere is 
removed to a great distance from the inferior metamere, and 
in which the two articulations are united only by a long and 
very thin peduncle, proves very clearly that the metameres 
are true articulations produced by incomplete division. Just 
the same thing (that is to say, the great individuality of each 
metamere, united only by a fine peduncle) occurs also in Cory- 
morpha pendula, Agass. +; ; only the individuality of the meta- 
meres is unequal, being easy to see in some Hydroids (such 
as Vorticlava, Stauridium, and Cladonema), and more or less 
effaced in others with many metameres (Zanclea, Coryne, 
&c.). But in any case we must regard the articulate type as 
a small colony. 
The number of individuals in such a community may be 
* Each tentacle (each antimere), or rather each pair of antimeres, is 
nothing but an axis vertical to the axis of the body ( he ite or primary 
axis), which is ordinarily called a secondary axis. The different types of 
Hydroids have 1, 2,3, 4n,....secondary axes, 2. e. 2, 2X2, 2x3, 2x4, 
2x n tentacles. 
+ Mon. Brit. Hydr. vol. ii. pl. xxiii. fig. 2, d. 
{ Contrib. Nat. Hist. Un. States, vol. iv. pl. xxvi. figs. 14 and 17. 
