198 Dr. R. Greef on the Structure and 



contractile), and are only transitorily united two upon one 

 stalk during division [VorticeUa)^ or the stalk rises, by con- 

 tinual and generally dichotomous division, into an arborescent 

 form, upon the terminal ramifications of which the individuals 

 are united into a colony which is usually very numerous. The 

 peduncles of these stock-forming Vorticellge are either con- 

 tractile (Carchestumj Zoothamnium) or rigid {Ejnstylis^ Oper- 

 cularia). The mode of ramification of the stock is very mul- 

 tifarious, and is often characteristic of the diiferent genera and 

 species ; so that it might with advantage be made use of for 

 systematic discrimination. A remarkable difference of this 

 kind in the ramification of the peduncle is presented, for 

 example, in the accompanying figures, between Ejnstylis fla- 

 vicans (PI. XV. fig. 1 &c.) and the Zootliamnium discovered 

 by me in the North Sea (PL XIV. fig. 6 &c.). Whilst in 

 Epistylis flavicansa. regular dichotomous ramification ascending 

 from the stem occurs, in the Zootliamnium in question the 

 shortly pinnate branches are placed alternately upon a common 

 shaft. Between these two very different stock-formations 

 there are, however, a number of others, which, as already 

 remarked, are more or less characteristic of the general habit 

 of the species under consideration. These, however, are almost 

 exclusively confined within the dichotomously expanded tuft- 

 or umbel-form ; the alternating branch-form has as yet been 

 observed only in stocks of marine Zootliamnia'^ . 



With regard to this latter genus a remarkable peculiarity 

 must here be mentioned, which is also characteristic of its 

 external habit, namely the frequently very remarkable differ- 

 ence in size of the individuals of the stocks. Individuals may 

 attain five or six times the size of the others, or even still 

 more ; and these then, especially when they are closed, project 

 from the majority of smaller individuals like lumps. Some- 

 times there are only one or a few of these lumps, but sometimes 

 a comparatively large number (PI. XIV. fig. 5). They may, 

 however, be entirely deficientf ; but this, according to my 

 observations, must be regarded as exceptional in the marine 

 forms in question. We shall revert hereafter to this form of 



* These alternating stocks were first observed by Ehrenberg in a species 

 discovered by him in the Red Sea, and named Zoothamnium niveum (Die 

 Infusionsth. kc. p. 289, pi. 29. fig. 3), then by Claparede and Lachmann 

 in Zoothamnmm altertums fi-om the North Sea on the Norwegian coast 

 (Etudes sur les Infusoires, ser. 1, p. 103, pi. 2. figs. 1-4), and finally by 

 myself in the form which I frequently saw at Ostend and other places on 

 the North Sea (PI. XIV. figs. G & 7), which is probably identical with 

 Z. alternans, but perhaps also with Z. niveum. 



t It is very remarkable that m Zoothamnium arhuscula, noth withstanding 

 the numerous examples which he examined, Stein entirely missed the 

 lump-like animals, which Ehrenberg observed and figured in this species. 



