244 M. C. Mereschkowsky on the Hydroida. 
cause has induced in the organism the tendency to grow 
constantly in length by incomplete transverse division; and 
if it happens that this cause acts for a long time, through a 
long series of generations, it is easy to understand that this 
tendency may acquire so great a persistency, and may become 
so powerful, that it will continue to manifest itself even after 
the disappearance of the original cause. Considered from this 
point of view, the law of physiological inertia appears simply 
to be a particular case of another more general law—the law 
of heredity ; and I believe that if we apply this law (without 
which the phenomena of the Annelida are perfectly obscure 
and incapable of explanation) to the group of Hydroids, and 
especially to the articulate type, we shall attain the possi- 
bility of explaining and understanding the appearance of such 
forms as Coryne pusilla and Gemmaria implexa. 
In the articulate type there is a peculiarity which is very 
interesting, especially because it can be very clearly explained, 
and to which I wish to call attention, namely the form of the 
tentacles. One of the most characteristic features which 
always accompany the law of metamerism in the Hydroids, is 
the capitate form of the tentacles, which in this case, are 
always very short (figs. 1, 4). This peculiarity of the articu- 
late forms is especially observable in the species with nume- 
rous metameres, in which the tentacles are excessively short. 
There are very few exceptions to the rule that articulation is 
combined with the capitate form of tentacles; and nearly all 
these exceptions can be perfectly well explained. 
In seeking to explain this fact, and to find the cause of its 
occurrence (raison d’étre), we must first of all call attention to 
the coexistence of the two facts, articulation and capitate ten- 
tacles, and inquire whether this singular and invariable coex- 
istence is not due to a causal relation between the two facts. 
It is more than probable that this is the case ; and, as we shall 
see immediately, it is the articulation that is the cause 
of the form of the tentacles. 
The articulate type has no doubt originated from the non- 
articulate type with 4, or, in general, 2 x x filiform tentacles. 
The tentacles of this general type (2x) are:—l, usually 
rather long and slender, not capitate, endowed with great con- 
tractility ; and, 2, covered over all their surface with a quan- 
tity of thread-cells. Such an organization is adapted to sub- 
serve two functions at once—namely, (1) seizure of food, 
and (2) defence against enemies by means of the veno- 
mous thread-cells. When the Hydroids furnished with meta- 
meres began to be developed from this type thus constructed 
and non-articulate, and, at the same time, the length of the 
