394 ORDER FLAGELLATA-EUSTOMATA. 



observation of almost constantly recurring currents in the surrounding water. The 

 practical demonstration of the possession by these animalcules of a single terminal 

 flagellum, of their enjoyment of a solitary free-swimming and social colony-building 

 existence, and of their entire structural correspondence during the former phase with 

 the zooids of the genus Euglena, was arrived at by the author in the winter of the year 

 1877, in connection with the Colaclmn stentonniiin^ in part, of Ehrenberg, and the new 

 species here described under the title of C. Steinii. The observations then made are 

 substantially confirmed by Stein's figures and accompanying indices, illustrating other 

 allied species, published in November 1878. Certain points observed by the author 

 have, however, apparently escaped the attention of Professor Stein, and are now 

 recorded for the first time. Chief among these it must be mentioned that in no 

 instance does that authority indicate the possession of a flagellum by any members 

 of the sedentary colony-stocks, the animalcules, according to his figures, after their 

 first attachment losing this organ, and their offspring produced by repeated longi- 

 tudinal fission of the primarily attached zooid, which remains affixed to the extremities 

 of the branching pedicle, not developing any flagellate appendage. As shown, 

 however, in the author's account and illustrations of the species quoted, the flagella 

 are developed, though difficult to detect, by both the motile and sedentary zooids, 

 disappearing through absorption as a preliminary only to the act of encystment 

 and sporular reproduction. 



The two cycles of existence manifested in the representatives of this genus find, 

 with the exception of Chlorangium, no parallel among the entire class of the Infusoria. 

 As in both the sedentary and motile conditions the reproductive faculties would 

 appear to be represented — the power of multiplying by ordinary fission being retained 

 by the natatory zooids — this generic group may be further cited as undoubtedly 

 yielding a novel instance of the interesting phenomenon of the alternation of gene- 

 ration. Whether genetic reproduction, accomplished by the conjugation of two 

 independent zooids, obtains during the sedentary or natatory phases, is an interest- 

 ing problem that yet awaits solution. The remarkable fixation of the locomotive 

 zooids of Colacuim by their oral end, and development into stalked sedentary 

 organisms, in which the characteristics of their natatory phase are almost completely 

 masked, will scarcely fail to recall to mind the singular and somewhat parallel 

 metamorphoses exhibited by the Cirrhipede Crustacea. 



Colacium arbuscula, Stein. Pl. XXI. Fig. 33. 



Sedentary zooids elongate-ovate, about twice as long as broad when 

 extended, shortly pyriform or subglobose when contracted, grouped at about 

 the same level at the extremities of a slender, erect, smooth and even, 

 dichotomously branching pedicle, the basal stem of which equals or con- 

 siderably exceeds the height of an extended zooid, the secondary and 

 succeeding ramifications being shorter ; parenchyma enclosing comparatively 

 few largish ovate chlorophyll-corpuscles. Length of bodies i-iooo" to 

 1-800"; height of adult colonies 1-250". 



Hab. — Fresh water, on Rotifera. 



The examples of this species figured by Stein* are represented as attached to 

 the Rotifer AiinrcEa fissa of Mr. P. H. Gosse. No less than two adult colonies, 

 bearing respectively six and eight zooids, and four smaller ones, are thus shown 

 growing upon one specimen of this minute rotifer, whose movements in the water 

 must have been much impeded by the accession of a living burden whose total bulk 

 considerably exceeded that of its own body. No illustrations are given or references 

 made to the free-swimming zooids of this type. 



'Infubionsthicre,' Abth. iii., 1878. 



