06 Proceedings of the 



polypary and its associated organs ; all combine to form an 

 object of the highest interest, and indicate that in this fixed, 

 yet travelling zoophyte we have a type of structure transitional 

 between the dendritic hydroidae and the more highly organized 

 acaleph. In the simplest acalephoid form, such as the medusoid 

 of Campanularia (which is nothing more than an extension 

 of the polypary specially organized for independent and mo- 

 tile life), we have (as in Hydractinia) an expanded polypary, 

 represented by the umbrella, and permeated by vascular tubes, 

 from the confluence of which last spring, at the centre, the 

 single alimentary polyp; at the periphery the tentacular polyps, 

 various in number ; and between them the reproductive polyps, 

 represented by the sessile generative sacs. In the higher 

 Rhizostomes we find, as pointed out by Huxley, a multitude 

 of alimentary and reproductive organs, united by ramifying 

 tubes, which permeate a massive swimming polypary. In 

 other acalephee, such as Velella, Porpita and Ratraria, we pro- 

 bably have a still closer resemblance to Hydractinia, in the 

 development of a corallum, represented by a horny plate, 

 which in the last of these medusae elevates the crest, serving 

 as a sail by which the floating mass is propelled before the 

 wind along the surface of the waves. 



33. In conclusion, I must mention that Hydractinia is in- 

 fested by a small species of Eolis (Eolis nana), which peels 

 off the polypary with its rasp-like tongue, and devours it, — pos- 

 sessed, I suppose, of some potent magic, which renders all the 

 formidable armament of its prey of no avail. Now, each of the 

 dorsal papillae of the Eolidse contains at its extremity a small 

 ovate vesicle, communicating, through the biliary sac, with the 

 digestive system, and opening externally by a minute aperture 

 at the end of the papillae. This vesicle is found crowded with 

 compact masses of thread-cells ; which masses, in Eolis nana, 

 consist of aggregations of small and large thread-cells, identi- 

 cal in size and shape with those of Hydractinia, — on which this 

 Eolis preys — not contained in capsules, but cemented together 

 by mucus. When we consider that each of the vesicles is in in- 

 direct communication with the stomach, I think we may, with- 

 out presumption, suggest a probability that the masses of 

 thread-cells found in Eolis nana are quasi faecal collections of 



