14 



AMERICAN HYDROIDS. 



radial. Of course this symmetry is often interfered with, particularly where the hydrotheca 

 has a curve' that is not in the vertical plane, as is the case with many species in which the hydro- 

 tlicca- curve forward as well as outward and upward, as Sertulur, II" jiinnata (Plate XXI, 

 fig. 10), and X. iillniiiii! (Plate XVIII, fig. 3). Again the implantation of the hydrotheca may 

 be upon the front rather than upon the sides of the stem, and thus we have a difference 

 between the front and back, as well as between the adcauline and abcauliue sides of the hydro- 

 theca as in the case of ILijiIraUninnin fn/n/fii (Plate XXXVIII, fig. 1), and in the Desinoscyphus 

 group of .^ i-tiiliirii(- (Plate I, fig. 7). That the bilateral symmetry is the result of the mechanical 

 causes referred to is rendered all the more probable from the, fact that the primary hydranth of 

 many species of sertularians is seen to be radially .symmetrical if observed at a very early stage 

 in its developement. If we examine very young specimens of /Serfatlarella ni</<>i.t, for instance, 

 we find that the primary hydranth is at first mounted on a distinct pedicel like that of a campan- 

 ularian, and is radially symmetrical (see fig. 21). 



It very shortly, however, becomes bilaterally symmetrical by the production of a protuber- 

 ance on one side of its base, and this protuberance shortly gives rise to a second hydrotheca. 

 This seems to indicate that radial symmetry is the original condition and bilateral symmetry a 

 later acquisition in the group, brought about by mechanical causes relating to the necessity of 

 adjusting the sessile hydrotheca! to the hydrocaulus. The condition of affairs described above 

 is true only of those species having alternate hydrotheca 1 . When they 

 are strictly opposite, the two terminal hydrothecse are produced together, 

 and hence such species produce primitive pairs of hydrothecffi that are 

 necessarily bilateral. 



In almost all species of sertularians the several hydrotheca? of the 

 mature colonies are substantially alike, the only notable exception being 

 in the genus Pasythea, where the hydrotheca? are in groups of pairs and 

 no two pairs in a given group are alike either in form or size (see Plate 

 XIII, fig. 4). 



With the exception of the genus SeLaginopsis all of the American 

 sertularians conform to the bilateral plan not only in regard to the shape of 

 the individual hydrotheca 1 , but also in the arrangement of the hydrotheca 1 

 on the stem and branches (see fig. 32). This is due to the fact that the 

 hydrothecw are arranged in two usually opposite rows. If a branch of 

 such a species should be split vertically from front to back it would be 

 divided into two equal and symmetrical parts. When the hydrotheca' are, 

 alternate, these two parts would be similar but not symmetrical. The same, 



is true in Hydrall/mania, where the hydrothecse are all in a single row but have their distal ends 

 bent alternately to the right and left. In Selaginopsis the hydrotheca 1 are arranged in more than 

 two rows, in one case, S. ilici /*</ v' ( ///.y, there being ten longitudinal rows. In this genus there 

 is not only the regular vertical arrangement of hydrotheca 1 , but a spiral arrangement as well. 



The hydrotheca 1 vary greatl}' in different species both in size and shape. In general they are 

 much deeper in proportion to their diameter than those of the Plumularida?. Perhaps the most 

 common form is more or less tubular, with the distal end bent to one side, as in S< /-fi/////-//t /nnnilii. 

 Most species of the genera Sertularia, Xi/iif/m-ii/ii/, and Tlmiaria have hydrothecse of this type, 

 which reaches its most perfect form in such species as Si/)if],,;-nn,i rectum (Plate XLI, fig. 2). 



Sometimes the tube is not bent, and the hydrotheca becomes an almost perfect cylinder, as in 

 X, ffnlui-,/!,/ funiioxii (fig. 22) or Synthedum eylhulrieutii (Plate XLI, fig. 7). Again it may be 

 so short, truncated, and expanded at the base as to resemble the frustum of a cone, as in Xt-rtii/ii- 

 r.l/n Jim-tin nl>; (Plate XXVII, fig. 5). The cylindrical hydrotheca?, like all others among the 

 Sertularidse, vary greatly in the extent to which they are immersed in the hydrocaulus, sometimes 

 being attached to the latter by their extreme base only, as in S. yi/adntta (fig. 23); or it may be 

 immersed to the margin all around, as in Sertnl<ir<-Ua distans (Plate XIX, fig. 6) or ,V,/y ///,//, II, i lutn 

 (Plate XVIII, fig. 10). Every possible intergradation between these two extreme.-, can be found. 

 Sometimes great variation occurs in 'a single colony, as in the case of Sertularella mayellanica 



Fig. 21. VLTY ynniiK liulrn- 

 ther:] ul' ,N i hti'in flu i //<,,-ir. 



showing radial symmetry. 



