of Hydroids from the White Sea. 223 



(PI. V. %. 3, d\ d\_ d\ d% d\ d% and figs. 12, 13 of PI. 

 VI.). I feel no hesitation in ascribing to these eight sacs 

 the functions of the sexual organs in which are contained 

 spermatozoa and ova. My determination of them rests on the 

 following grounds: — 1. The medusffi, on account of the sim- 

 plicity of their organization, cannot have any other organs 

 than the radiating canals, the manubrium, the tentacles, and 

 the sexual organs ; and as these sacs can be neither of the 

 three first-named organs, it is evident that they must be con- 

 sidered as the sexual organs. 2. On account of the analogy 

 existing between this hydroid and several species of the Gery- 

 onidaj (for instance, Aglaura Peronii)^ in which the sexual sacs, 

 by their form and dis])osition, bear a great resemblance to the 

 sacs Avith which we are at y)resent occupied. 3. On account of 

 their contents consisting of two sorts of granules — one set being 

 0'00047"' in diameter, and the otliers (evidently produced by 

 the division or the segmentation of the first) being about 

 0"000235"'. These small granules, obviously spermatozoa in 

 an immature state, are formed, as is well known, hj the division 

 of the mother cell into several segments. The existence of 

 such sacs is not often to be observed in the Hydroids, and 

 the originality of the Hydroid now under study is thereby 

 increased. The structure of the medusa is set forth very 

 conspicuously by my two figures (PI. V. fig. 3, and PI. VI. 

 figs. 12, 13), drawn from nature by the aid of a camera lucida, 

 and not in the slightest degree diagrammatic. 



The young gonophores show a little difference in their 

 construction from those just described. The cross section 

 shows only four sacs (PI. VI. fig. 14) ; but if we take these 

 gonophores at a more developed state, the cross section under- 

 neath shows eight sacs (PI. VI. fig. 12) — in the centre four 

 (PI. VI. fig. 13), but with an interruption at the middle, and 

 in the upper part four without an interruption. From this 

 it follows that at the beginning there appear four sacs, arid 

 this number afterwards, by longitudinal division, increases 

 to eight. Moreover the sexual sacs present an interesting 

 peculiarity in their division ; that is, the more advanced divisions 

 of two sacs lie opposite each other ; and this appearance 

 was noticed by me not once, but on every cross section I made 

 (PI. VI. fig. 7, a, 5). It appears to be one of the many facts 

 showing that the fundamental number of hydroids is not 

 four, as is usually admitted, but only two, and that all the 

 remaining numbers are nothing else but 2 x «, from which it 

 follows that six and ten are not an exception to the rule, but 

 merely 2x3 and 2x5. For such facts, pointing to the 

 number 2 as the basis, refer for instance to the history of the 



