103 



V, 



STUDY XXI. THE PLUMULARIDAE. 



None of our Zoophytes are more graceful than are the Plumu- 

 laridae their form justly entitles them to the name they bear, so 

 exquisitely beautiful are their feathery plumes, borne sometimes on 

 the green blades of the sea-grass (Zostera), or on the stout brown 

 edges of Fucus, or even on the bare rocky sides of some sheltered 

 pool. 



Very nearly related in general form to the Sertularidae, this family 

 differs in two important points : in the former, the polyps are 

 arranged along both sides of the branches, in the latter, a row is 

 found on one side only ; again, among the Plumularidae, minute and 

 degraded individuals are found, known under the name of nemato- 

 phores organs entirely wanting among the Sertularidae. In the 

 species here figured, one occurs just beyond, and another below each 

 hydrotheca, whilst two are located in the axils of the stem and 

 branches. Considerable mystery attaches to these structures, their 

 function being still problematical. In form they are tiny chitinous 

 cups wherefrom project extremely extensile sarcodal threads. The 

 term sarcotheca is applied to the cups ; sarcostyle to the thread. 

 The latter, though extremely slender and tenuous, and not unlike 

 the gigantic pseudopodia of some monstre Foraminiferon, are truly 

 cellular, composed of an outer ectodermal layer sheathing an endq- 

 dermal core. A remarkable feature is the projection of pseudopodia- 

 like processes from the surface of the sarcostyle. The cell-walls are 

 all extremely tenuous and thus capable of great elongation. The 

 extensile power of these threads is indeed marvellous. Fig. VI., 

 PI. VI., gives but a faint idea of this. When active, they may be 

 seen winding and coiling with snake-like litheness around the hydro- 

 thecae and the branches. Especially active are they in the neigh- 

 bourhood of dead polyps, and here perhaps is their sphere of useful- 

 ness, in the removal of decaying matter a theory strengthened by the 

 presence of foreign particles in certain amoeboid cells of the surface. 



The great reproductive capsules or gonangia are especially well- 

 developed in this species, crowding the lower end of the main stem 

 (hydrocaulus), the lower branches, and especially the creeping 

 stem or stolon that connects the various plumes of the commonwealth. 

 In these the reproductive cells mature and undergo segmentation, 

 but, thanks to Weismann's researches (Die Enstetnung der Sexual- 

 zellen bet den Hydromedusen) we now know that in this family, the 

 Plumularidae, they do not originate within the gonangia, but arise in 

 the endoderm of the stem, whence they migrate to the gonangia as 

 into incubatory pouches. Figs. 3, 4 and 5, PI. VI., illustrate these 

 protective sacs in various stages, and show how the sexual cells con- 

 gregate in a sphaerical mass a false ovary. 



