248 M. C. Mereschkowsky on the Hydrotda. 
support of the explanation above given of the forms with 
capitate tentacles. I refer to the blastostyles of the genus 
Hydractinia. tis well known that the gonophores, or sexual 
individuals, appear upon the surface of the body of the tropho- 
somes, or nutritive individuals, which are furnished with 
several filiform tentacles. When these gonophores appear, 
the individual upon which they are seated, and which is then 
called a “ blastostyle,” becomes much thinner and smaller (the 
material of the animal being absorbed by the sexual bodies), 
the mouth closes, and the tentacles (which, from this moment 
lose their importance as organs subserving the purpose of 
nutrition, since the mouthless individual cannot feed) become 
shorter and shorter and more and more insignificant. Soon 
we can only perceive a few knobs or tubercles furnished with 
a great quantity of thread-cells, greatly resembling the dila- 
tations with which the capitate tentacles are furnished. At 
the same time these tentacles or tubercles only retain the 
function of defence from enemies. 
But I have said that there are exceptions to the rule that 
articulation is associated with capitate tenta- 
_ cles, and that these exceptions are not con- Fig. 7. 
tradictory to the explanation that I have 
given; on the contrary, it is possible to |] \ } 
explain these exceptions only by admitting 
all that I have said above. 
_ There are some forms, evidently belonging 
to the articulate type, which do not possess 
capitate tentacles, but, on the contrary, have 
those organs slender, filiform, and very long. \ 
For example, Clava*, Cordylophora, &c. \ | 
(especially Cordylophora) have tentacles 
longer than in any other species. This is h A 
to be explained as follows :—In becoming ( /] 
developed into the articulate type the Hy- ‘ 
droid became more and more elongated, NI 
whilst the tentaclesremained thesame, which 
rendered them relatively shorter; and it is | 
precisely this that induced their capitate 
form. But if we assume that all the time 
the elongation of the tentacles proceeded — (ordylophora 
side by side with the elongation of the body, lacustris. 
we shall see that their original significance, 
as aiding in alimentation at the same time as for defence, 
* Especially Clava leptostyla, Ag. (A. Agassiz, Illustr. Cat. Mus. Comp. 
Zool. ii. p. 170, fig. 274). In the cases in which the tentacles are not 
too long we may admit that these species have only become articulate 
