M. C. Mereschkowsky on the Hydrovda. 245 
whole animal increased considerably, so that the long, slender, 
original tentacles, which previously extended beyond the apex 
of the body (where the mouth is placed), became relatively 
shorter, they would no longer reach the buccal orifice, which 
would deprive them of all power of acting as organs of nutri- 
tion. This must certainly take place, especially with the 
lowest tentacles. The part which they performed being thus 
diminished, and their significance in the economy of the 
animal changed, the tentacles would no doubt undergo, if not 
complete atrophy, at least a consi- 
derable diminution in their develop- Fig. 6. 
ment. This is, in fact,what we ob- 
serve. In such articulate forms as 
Cladonema radiatum (fig. 6), for ex- 
ample, which is furnished with two 
very distinct metameres, we remark 
that the four lower tentacles, belong- 
ing to the inferior metamere, are too 
short to reach the mouth, and conse- 
quently cannot possibly assist in the 
process of nutrition; at the same time 
they are much less developed, much 
shorter and more delicate than the 
other four tentacles belonging to the upper metamere, which 
can very easily reach the mouth. 
The form with four metameres (Stauridium productum) 
shows us the same thing. In this also the four tentacles of 
the inferior metamere are, to a very great degree, atrophied*. 
The same thing takes place in all the other articulate forms, 
even when the number of metameres is very considerable, as, 
for example, in Coryne pusilla and Gemmaria implexa. In 
all cases the inferior tentacles are less developed, half or one 
third of the length of those of the superior metameres, and the 
more they approach towards the basal extremity of the body 
the shorter they are, so that in the lowest regions the length 
of the tentacle often does not exceed its thickness; but the 
superior tentacles, as well as the inferior, are comparatively 
much shorter in the articulate than in the non-articulate type. 
This atrophy of organs evidently depends upon a diminution 

* We know no form governed by the law of metamerism having 5 
metameres ; but it is easy to see that such a form must once have existed, 
and that it, perhaps, still exists in some little-investigated sea. If it be 
found some day, we may predict with great probability, from the evidence 
of the forms with 2 and 4 metameres, that it will also have 4 tentacles 
(or 2X) belonging to the inferior circle more atrophied than the rest. 
The genus Triridium, to which this hypothetical Hydroid must belong, 
is represented in fig. 1. 
