REPORT ON THE HYDROIDA. 
XVII 
abundant in the tentacles of both hydranth and Medusa. In such tentacles as possess a 
terminal enlargement or capitulum (Cory tie, Syncoryne, &c.) they are especially accumu- 
lated in this enlargement. In the marginal tentacles of many Hydromedusse, they 
form condensed verticillate groups regularly distributed from distance to distance along 
the length of the tentacle, to which they give a moniliform character. In such cases 
the tentacle usually terminates in a spherical enlargement which is loaded wit|i 
thread -cells. 
In certain other Medusae, which are also derived from hydroid trophosomes, 
we meet with special arrangements of the thread-cells. Thus in the Medusa of 
Gemmaria implexa 1 we find four superficial pyriform chambers extending from the 
umbrella margin in the outer ectodermal wall of the umbrella, and filled with thread- 
cells which doubtless originate in the walls of these ectodermal chambers, and thence 
apparently fall into their cavities. The marginal tentacles of this Medusa give rise along 
their entire length to filaments endowed with great powers of extension and retraction, 
each carrying on its summit an oval ciliated sac filled with thread-cells. In the Medusa 
of Poclocoryne earned each of the four lobes into which the mouth of the manubrium is 
here divided carries a pencil of non-contractile filaments, each of which bears on its 
extremity a solitary capsule resembling a enidocyst, with its cnidocil and contained 
thread-cell. We can scarcely avoid a comparison of these naked pedunculated 
cnidocyst-like bodies with the cnidocysts as they elsewhere occur embedded in the ecto- 
derm with their basa] filiform prolongations. 
In Sertularia exserta (PL XXVII. fig. 1«), one of the new forms obtained by the 
Challenger, small thread-cells are accumulated in a little cushion-like prominence at the 
base of every tentacle (figs. 16, lc). In this species the hydranth presents the very 
exceptional character of remaining in a state of habitual extension beyond the protective 
covering of the hydrotheca, and the batteries of thread-cells thus disposed would seem to 
have as their object a compensation for the loss of the protection which in most other 
Calyptoblastic Hydroids is afforded by the hydrotheca. 
In certain Hydromedusse (Trachomedusse and Narcomedusae, see below, p. xxix) 
thread-cells are accumulated on the umbrella margin which they surround in the form of 
an urticating ring, while in most of these Medusae accumulations of thread-cells 
forming narrow urticating patches stretch from the umbrella margin in a meridional 
direction to the roots of the tentacles, which here spring from the dorsal surface of the 
umbrella at some distance from the margin. 
Ganglion Cells. — Quite recently Jickeli has called attention to certain ectodermal 
cells which he has found widely distributed in the trophosome of many Hydroids, where 
they lie scattered between the deeper ends of the other ectodermal cells. 2 He describes 
1 Gymnoblastic Hydroids, p. 291, pi. vii. 
2 Carl F. Jickeli, Morphol. Jahrb., Bd. viii. 
(ZOOL. CHALL. EXP. PART LXX. 1888 .) 
Aaaa c 
