22 RESPIRATION AND CIRCULATION. 



transparent membrane in the form of a cup or calyx (PI. II, fig. 24 ; V, fig. 5 ; IX, fig. 7, m). 

 This cup is adherent to the hack of the tentacula, and its margin is in most instances pro- 

 lono-cd more or less upon each tentacle, as a narrow triangular process, so as to present a 

 sort of scalloped or festooned appearance ; the festooning of the margin is most marked 

 in Fredericella : in some species of Flumatella it is scarcely perceptible. A high power of 

 the microscope, and carefully adjusted illumination, will enable us to detect in the calyciform 

 membrane certain delicate anastomosing lines. These appear to indicate the surfaces of 

 contact of cells of which the membrane would thus seem to be composed ; they are par- 

 ticularly evident in Cristatella. It is curious enough that the calyx should be exactly 

 coincident with the presence of an epistome in all the fresh-water Polyzoa; unless Uniaiella 

 should prove to be an exception, which is not likely, though we are not yet sufficiently 

 acquainted with the structure of the genus to include it in this generalization. In no marine poly- 

 zoon has a calyx yet been detected, unless we admit the by no means improbable supposition that 

 it enters partly into the composition of the calyx-like cup which surrounds the base of the 

 tentacles in Pedicellina, the only marine genus in which an epistome is also represented.* 



The perigastric space, and interior of the tentacula and lophophore, all freely communicate 

 with one another, and are filled with a clear fluid, in which float numerous particles of very irre- 

 gular form and size. In this fluid may be observed a constant rotatory motion, rendered apparent 

 by the floating corpuscles as they are whirled away under the influence of the currents. That 

 the fluid thus contained in the perigastric space, and thence admitted into the tentacula, con- 

 sists mainly of water which had obtained entrance from without, there can, I think, be little 

 doubt, and yet I have in vain sought for any opening through which the external fluid can 

 gain admittance to the interior. I have allowed the transparent genera Cristatella and 

 Lophopus to remain many hours in carmine without being able to detect a single particle of 

 this pigment in the perigastric space, though I have seen this space rapidly empty itself on 

 the removal of the animal from the water, and again fill on restoring it to its natural element. 

 Van Benedent believed that he had detected in Alnjonella apertures, which he names 

 " bouches aquiferes," at the base of the tentacula ; but this distinguished naturalist is certainly 



given to the stalk, and these motions, when witnessed in a living and active group of Pedicellirue, 

 present an appearance in the liighest degree novel and interesting. 



The stalk of Pedicellina must be viewed as homologous with the posterior part of the cell in 

 the unstalked forms of Polyzoa. It is simply this portion of the cell become so much constricted as 

 to be no longer capable of containing the polypide, which is in consequence pushed onwards into the 

 wider portion which now constitutes the proper cell. The muscles of the stalk have their represen- 

 tative in the muscles developed in the walls of the cell of Polyzoa. 



Between the oesophagus and rectum, but separated from the latter by the whole mass of the 

 generative system, is the nervous ganglion, close to which, and lying between it and the oesophagus, is 

 a peculiar organ in the form of a minute tubular cavity clothed with actively vibrating cilia. I was 

 unable to follow this organ through its whole extent, or determine its exact relations, as it appeared to 

 lose itself beneath the opacity of the surrounding structures. Its situation would suggest the proba- 

 bility of its being an organ of sense. It is possible, however, that it is a portion of a more extensive 

 system of tubes, a supposition which some appearances seem to warrant, and then we might perhaps 

 view it as indicating the presence of a water-vascular system. 

 * See previous note, 

 t Quelques Observations sur les Polypes d'eau douce, 'Bull, de I'Acad. Roy. de Bruxelles,' 1839. 



