M. C. Mereschkowsky on the Hydroida. 241 
has the least normal tendency to the production of meta- 
meres*, 
The number of metameres is very variable in different genera 
and species. We know several Hydroids (as, for example, 
Cladonema radiatum) which have only two metameres, usually, 
in this case, very distinct and well marked. Up to the pre- 
sent time we do not know a single Hydroid which has three 
distinct metameres; but we know one with four very - 
clearly developed; this is Stawridiuwm productum, which I 
have found in the White Sea. After it come the forms 
which have more than four metameres ; but in these cases they 
are not very distinct, and their number is no longer constant, 
but varies with the age and development of the individual. 
At the same time, this variation in the number of metameres 
takes place within certain limits; for there are species in which 
the metameres never attain the great number met with in 
other species. 
Among the forms which are very rich in metameres I can 
cite several, but especially Coryne pusilla and Gemmaria 
impleaa, in which we may see a very great quantity of meta- 
meres. Itis true that in such cases it is impossible to fix 
clearly the boundaries of two metameres, and that it is often 
impossible to decide whether two tentacles belong to one or 
to two different metameres ; but nevertheless it is easy to see 
that we have to do with the same articulate type as in Stau- 
ridium and Cladonema ; only here the order of arrangement of 
the tentacles, in consequence of their great number, has be- 
come very much effaced. 
As to the nature (das Wesen) of the law of metamerism, the 
cause which has produced the articulate type among the Hy- 
droids, I think I am justified in explaining it in the following 
manner :—T'he Hydroid, in consequence of a great abundance 
of nourishment, or from some other cause unknown, began to 
grow in the direction of the primary axis of its body. Growth, 
as we know, does not differ generically from the process of 
multiplication ; the latter is only a particular case of the 
former ; and the two processes depend greatly upon each 
other. 
The growth of the Hydroid beyond its specific limit causes 
* A single anomalous fact is known to me in the Thecaphora, in which 
there appears a tendency, although a very feeble one, to take on the arti- 
culate form. This is the Clytia poteriwm, Agassiz (fig. 2), in which one 
hydrotheca is placed above another, which has produced it, no doubt, by 
division. But this case can only be regarded as an anomaly, the nor- 
mal individuals never having any trace of metamerism. This anomaly 
has been described by Agassiz, ‘Contributions to the Natural History of 
the United States,’ iv. p. 303, pl. xxix. fig. 1. 
