XVI 
THE VOYAGE OF H.M.S. CHALLENGER. 
continuous with the longitudinal muscular fibrillse. Lying between the caudal prolonga- 
tions of these superficial cells, are other cells destitute of prolongations, and forming a 
tissue to which Kleinenberg gives the name of interstitial tissue. 1 It is among these 
interstitial cells of Hydra, and among cells which, by their deep position in the ectoderm 
and freedom from connection with the muscular fibrillse, correspond to them in other 
genera, that we find the cnidocysts or cells within which the thread-cells are formed. 
The thread-cells are developed out of the protoplasm of the cnidocysts in a way not 
yet determined, and without the direct participation of the nucleus of the cnidocyst, 
which after the development of the thread-cell still continues visible in it. Jickeli, from 
some observations which he has made on the thread-cells of Hydra, concludes that the 
axial tube is at first present as an external extension of the walls of the capsule, and that 
it subsequently becomes internal by invagination. 2 
Since the thread-cells in order that they may exert their proper function must be 
among the most superficial cells of the ectoderm, a migration from the deeper to the 
more superficial parts of this tissue-layer becomes necessary. It will be afterwards 
shown that the faculty possessed by certain cells, both of the ectoderm and endoderm, 
of wandering from one part of the Hy droid body to another, is now a well-established 
fact. No light has yet been thrown on the mode of formation of the cnidopods or 
filiform processes which are sent off from the bases of the cnidocysts. 
That the thread-cells serve as weapons of defence and offence is now generally 
admitted. Their whole structure, and the phenomena which they present when 
called into action, are all in favour of the view which would assign to them such 
an office in the economy of the animal. Their benumbing and even fatal action on the 
animals with whose surface they come in contact has been too often noticed to allow of 
any doubt on this point. Semper 3 has described a gigantic Plumularian, a native of the 
East Indian Archipelago, which attains nearly the height of a man, and which on account 
of its formidable stinging properties is held in dread by bathers. Kirchenpauer has 
identified Semper’s Plumularian with an Aglao'phenia to which he assigns the specific 
name of philippina, probably identical with Aglaophenia macgillivrayi, examples of 
which have been obtained by the Challenger, and have been described in the first part 
of the present Report (Plumularidse, p. 34, pi. x.). That the stinging property which 
can thus make itself so severely felt even by the human subject must here reside in 
the thread-cells will scarcely admit of doubt. Indeed there is no other part of the 
Hydroid to which it can with any reason be attributed. 
The thread-cells of the Hydroida, though almost exclusively confined to the 
ectoderm, are by no means uniformly distributed in it. They are as a rule most 
1 N. Kleinenberg, loc. cit. 
2 Carl F. Jickeli, loc. cit. 
3 C. Semper, Reiseberieht, Zeitschr. f. wiss. Zool., Bd. xiii. 
