246 M. C. Mereschkowsky on the Hydrovda. 
of their utility to the organism ; for in the articulate type the 
tentacles, instead of fulfilling two functions at once, only per- 
form one, namely that of defence against enemies. At the 
same time it has become possible for the organ to adapt itself 
better to the single function of defence than before, when it 
required also to capture food; it has attained the possibility 
of retaining the characters which are only useful for defence 
and which are even injurious to prehension. It is precisely 
this possibility of adapting themselves to the single function 
of defence that is the cause of the tentacles in the articulate 
type being very short and capitate at their extremity. 
Imagine now a Hydroid reposing after a full meal, with 
its tentacles quietly expanded in the water and gently moved 
to and fro by the waves. When any enemy approaches it 
with hostile intentions and is inclined to attack it, the assailant 
must most certainly strike against the ends of the tentacles 
before it can touch the body of the Hydroid. Upon the effect 
produced by this first contact with the ends of the tentacles 
will depend all the subsequent actions of the enemy: if it 
receives a very strong charge it will be killed on the spot, 
or will make its escape as quickly as possible ; in the contrary 
case, when the pain caused by the thread-cells arranged in 
the ends of tentacles is too insignificant, the enemy may arrive 
at the very body of the Hydroid, which is then menaced with 
great danger. We see, therefore, that, for the purpose of 
self-preservation, it is very important that the first line of 
fortifications, so to speak, should be as strong as possible—in 
other words, that the ends of the tentacles should be as for- 
midably armed as possible, that there should be as many 
thread-cells as possible in these ends; for those which are 
placed in the other parts of the body and tentacles are not of 
equal importance for the purpose of defence. To fulfil all 
these conditions it is clear that the tentacles must be inflated 
at their extremities, in order that a great quantity of thread- 
cells may be accumulated in the enlargement. When once 
these tentacles have ceased to act as organs of prehension, it 
is no longer necessary that they should be long, fine, supple, 
and movable; this is why in the articulate type, at the same 
time that they acquire the capitate form, they also become 
much shorter than usual. 
It generally happens that in those cases in which the body 
becomes very much elongated it acquires great flexibility and 
the faculty of twisting about very briskly, and so assists the 
proboscis in the capture of food, whilst, on the other hand, 
this organ in such cases also becomes strongly developed and 
very mobile. This flexibility of the body consequently re- 
