S HYDROIDA 



here the stinging; cells are arranged in dense transverse belts round the tentacles so as to give these, 

 when distended, a peculiar transversely striped appearance. 



On the ground of filiform and claviform tentacles Kiihn (1913) divides the athecate hydroids 

 into the two principal groups of Filifera and Capitata. However, we find in the latter group also 

 instances of filiform tentacles; in the Pennariidce filiform tentacles occur together with claviform ones; 

 in the Tubulariidce, on the contrary, the claviform tentacles have disappeared, at any rate in the polyps 

 fully developed. Kiihn (1. c. p. 228), therefore, also makes the reservation that the tentacles are "dauernd 

 oder in der Jugend geknopft". Embryological studies 011 Corymorpha have, in fact, shown that the 

 actinula has claviform tentacles (Torrey 1907, Hartlaub 1907). However, this state of things cannot 

 be generalized as a matter of course to embrace the Tubularia; on the contrary the figures of Allman 

 (1872) show quite distinctly that the actinula in Tubularia larynx and Tubularia indivisa have filiform 

 tentacles, and inquiries into the Tubularia regalis in the Trondhjem Fjord have shown no trace of 

 capitate tentacles during the development of the actinula. Nor do here claviform tentacles occur in 

 full-grown polyps. 



But even though the definition of Kiihn must be characterized as erroneous so far, there is 

 another criterion showing that his division of groups is correct. A searching study of the very uemato- 

 cysts, shows, as a matter of fact, that the athecate hydroids fall into two large principal groups, cor- 

 responding to the Capitata and the Filifera stated by Kiihn. 



In the hydroids occur two characteristic principal forms of nematocysts (PI. I, figs 1 — 7). In all 

 the Capitata we find large oviform or almost wholly spherical nematocysts of the same principal type 

 as is often mentioned in the Hydra. These large nematocysts are always accumulated on the tips of 

 the claviform tentacles, as in the Coryue, while on the filiform tentacles of the Tubularia they are 

 more equably distributed all over the ectoderm of the tentacles. However, these stinging capsules are 

 also found elsewhere in the ectoderm of polyps, as is the case with Monocorync and Myriothcla. In 

 the last mentioned form the nematocysts, like those of Millepora, have developed dimorphicly; besides 

 the typical oviform nematocysts we find here a larger and more slender oval form; in general the 

 latter nematocysts are outnumbered by the oviform ones, but still they amount to a large percentage 

 of the total stock of stinging capsules occurring in the animal. In the Millepora this type of nemato- 

 cysts, judging from the figures in hand, is rather broad. 



The rest of the hydroids are distinguished by quite a different type of nematocysts. The pre- 

 dominant type is a very small, all but rod-shaped nematocyst, particularly occurring in the tentacles, 

 generally accumulated in belts, vertically on the tentacle axis, giving the tentacles, when wholly 

 stretched out, a peculiar transversely striped appearance, like that of the tentacles of the Eudendrium 

 recorded. Concurrently with this typical small nematocyst distinguishing all Filifera sometimes occurs, 

 finally, a somewhat larger form, as in the Eudendrium and the Sfylasterida. In the Eudendrium 

 Wrighti we find in the tentacles only small rod-shaped stinging capsules; on the contrary in the 

 basal whorl of stinging cells of the polyp body of the species in question the capsules are much larger, 

 though keeping a slenderly oval appearance. It is strange that this large type of the Eudendrium 

 and the Sfylasteridir should appear almost entirely consistent with the aberrant slenderly oval nemato- 

 cyst with the Myriothcla. The simultaneous occurrence of the type in so different and so highly 



