102 THE OCEANIC HYDROZOA. 



The general colour of the hydrosoma is a pale, delicate green, passing gradually into 

 a dark, indigo blue on the under surface. 



The ridge of the crest is tipped with lake, and the pointed end is stained deep bluish 

 green about the aperture of the pneuniatocyst. 



The bases of the large tentacles are deep blue ; the polypites deep blue at their bases, 

 and frequently bright yellow at their apices ; the velvetty masses of reproductive organs 

 and buds on the under surface are light green. 



This description applies to individuals whose hydrosoma is four inches long and 

 upwards. In young specimens, the form and colour are apt to vary much. 



The structure of the hydrosoma is best made out in very young individuals, small 

 enough to be placed bodily under the microscope, as, in consequence of the extreme 

 contractility of the walls of this organ in the larger ones, it commonly collapses and contracts 

 into a formless mass the moment an incision is made into it. 



In such young Phi/salia, of one fifth to three eighths of an inch in length (PI. X, figs. 1, 2), 

 the wall of the hydrosoma, which presents a minute, closely shut aperture or pore 

 at one extremity, is very obviously composed of two distinct membranes,^ the outer, or 

 ectoderm, being denser and more transparent, while the inner, or endoderm, is softer, more 

 opaque, and ciliated on its inner free surface. 



In the region of the pore (fig. 3), the endoderm is reflected on to the outer surface of an 

 ovate sac, which occupies only a comparatively small portion of the cavity of the hydrosoma. 

 Tiie walls of this pneuniatocyst are dense, thick, and elastic, and its inner surface is covered 

 with a thin layer of granular matter. The end of the pneumatocyst on to which the 

 endoderm is reflected, is in close contact with the ectoderm, and when viewed from within 

 presents a depression, surrounded by circular fibres, and towards which short, radiating folds 

 of the membrane converge, which corresponds with the pore visible upon the outer surface, 

 and is the inner end of the pneumatic aperture. By pressure, the air contained in the 

 pneumatocyst could be made to pass out through this aperture. 



In adult specimens, the outer crescentic aperture of the pneumatocyst is visible on 

 the upper surface of the pointed apex of the hydrosoma, about half an inch from the 

 end. 



The ectoderm is composed of parallel, elongated, cocUffiform masses, arranged perpendi- 

 cularly to the plane of its surface ; the endoderm of an outer layer of delicate (muscular r) 

 fibres about -r^-jX\ of an inch in diameter, invested by a granular, internal substance, which is 

 richly ciliated. 



The appendages are large tentacles, small tentacles, polypites, gonoblastidia, and repro- 

 ductive organs. 



The tentacular appendages arise from the outermost of the numerous protuberances 

 developed from the under surface of the hydrosoma, or what is in reality the representative 

 of the stem-like ccenosarc of other Fhysophorida:. 



The larger tentacles of large specimens sometimes attain a length of many feet, and are 



^ I do not find this structure distinctly alluded to by my predecessors. They often speak of an 

 inner membrane, indeed, but always mean thereby the wall of the pneumatocyst. M. De Quatrefages 

 refers to it in other parts of the organism, as the mucous layer. 



