GENUS DENDROSOMA. 841 



The type of the genus. Dendrosoma radians, may be said to represent one of the 

 most remarkable forms described in this treatise. Not only does it occupy an 

 isolated position as the only known compound member of the Tentaculiferous 

 class, but it is furthermore the only example throughout the entire Infusorial series 

 in which the individuality of the component zooids is so far obliterated that the 

 entire colony-stock maybe justly described as a homogeneous multicellular organism or 

 " syncytium." In many of the Flagellata and Ciliata previously described including 

 notably among the latter the compound Vorticellidae such as Carchesium and Epistylis 

 somewhat analogous colonial aggregates occur ; but in all of these the individual 

 zooids are distinctly isolated from each other, the genus Zoothamnium affording the 

 only instance in which they are more intimately united through the interposition of 

 a living tissue, as represented by the axial muscle-like element or fibrilla of the 

 branching pedicle. In Dendrosoma, however, not only are the component zooids 

 indistinguishably fused with each other, but a common nutritive fluid circulates 

 throughout the creeping rhizome and ramifying branches. An adult colony-stock of 

 Dendrosoma radians, such as is represented at PL XLVII. Fig. 17, presents, indeed, 

 an extraordinary resemblance to the colony-stock of some Hydroid zoophyte, such as 

 Coryne or Cordylophora ; and, notwithstanding the fact that the component zooids of 

 Dtndrosoma must be regarded as potentially unicellular, which precludes a direct 

 homological comparison with such higher metazoic types, the points of agreement 

 that can be cited are most remarkable. In both cases we find in the fully developed 

 organism a basal adherent stolon or rhizome, which gives origin to a greater or less 

 number of erect diversely ramifying stems, the hydrocaulus or hydrosoma of the 

 zoophyte and zoocaulon or zoocaula of the Acinetan, these developing in either case 

 at their extremities tentaculiferous zooids, the tentacles, it is worthy of remark, in 

 Coryne further resembling those of Dendrosoma in being capitate. It is found that 

 the analogy suggested extends even beyond these more obvious superficial cha- 

 racters. The processes and products of the reproductive phenomena singularly 

 coincide with each other, but in the case of Dendrosoma are, as might be anticipated, 

 much more generalized. The Hydroidea, as is well known, produce embryos of 

 two kinds, these being either simply ciliated larvae or Planula, or medusiform 

 gonozooids, which are, strictly speaking, natatory modifications only of the ordinary 

 alimentary zooids or polypites. In but few, if any, members of this class are these 

 two descriptions of embryos developed in the same species. In Dendrosoma radians, 

 however, as hereafter shown, reproductive bodies analogous to both of these struc- 

 tural types may be produced by the same colony-stock, free-swimming ciliated 

 germs being liberated from the more massive basal region of the erect trunks, while 

 more minute tentaculiferous embryos, developed by gemmation in membranous 

 capsules, corresponding with the gonothecae of the Hydroid polype, are produced 

 towards the distal extremities of the branches, in close vicinity to the alimentary 

 acinetiform zooids. All these points being considered, added to the evidence in a 

 similar direction previously submitted, see vol. i. pp. 104 and 105, it is difficult to 

 arrive at a conclusion otherwise than that the Acinetidae, more particularly as typified 

 by Dendrosoma, represent the archetypes or direct lineal ancestors of the Hydroidea. 

 If Mr. Levick's observations concerning the development of spermatic elements in 

 certain colony-stocks and ova in others, as hereafter described, should be confirmed, 

 thus demonstrating the existence of true generative reproduction on a dioecious 

 plan, the arguments here brought forward in favour of the advanced affinities of 

 Dendrosoma will be immeasurably strengthened. 



Dendrosoma radians, Ehr. PL. XLVII. FIGS. 16-22. 



Stolon depressed, repent, giving origin to a greater or less number 

 of erect tentaculiferous zoocaula ; zoocaula simple or variously ramified, 

 thickest at the base, tapering evenly towards their distal terminations, the 

 secondary branchlets or ramuscules produced by longitudinal fission, divari- 



