6 THE OCEANIC HYDROZOA. 



In the adult, this sac, which I shall term the ptwumatoci/sf, is sometimes open at 

 the apex {Phpcdia, Tildzopkijsd), and can communicate with the exterior hy a pore which 

 traverses the ectoderm of the pneumatophore. In other cases its apex is shut, but the pneu- 

 matocyst has other external openings (Fe/(?//a, Porpifa); and, lastly, in Ph^sophora,ForskaHa, 

 Agalma, Hallstemma, and Alhori/hia, no external opening at all has hitherto been discovered. 



In certain of these genera [Porshalia, Ac/alnia, Halisteiiima), the pneumatocyst appears to 

 be widely open below. I suspect that more careful examination would show that it 

 only becomes very thin ; but however this may be, it is a mistake to suppose that there 

 is any communication between the interior of the pneumatocyst and that of the ccenosarc.^ 



In fact, as I have particularly noted," in Physalia, Vdella, Bhizojjlij/sa, Phijsophora, and 

 Agalma, the endoderm of the pneumatophore is reflected on to the pneumatocyst, where 

 the latter is in contact with the walls of the pneumatophore, and completely invests it, 

 forming a loose bag over the apparent inferior aperture, in those genera which possess one. 



The pneumatocyst is thus firmly held in its position by the reflected endoderm, and 

 in some genera (Forskalia and Agalma) additional support is afforded by septiform processes, 

 which pass from the lateral walls of the pneumatocyst to those of the pneumatophore. 

 These were first described by Milne Edwards in his admirable Memoir on Slcjilianomia 

 {Forskalia) coniorta^ in which he says, that "the air- vesicle is open below, and letained 

 in a central position by membranous partitions disposed in a radiating manner, and stretched 

 between its parietes and those of the great pyriform cavity (of the upper end of the stem), 

 nearly in the same way as the mesenteries by which the alimentary canal is surrounded 

 in the Alcyonian polypes." 



I find no notice of these suspensoria in the works of either Vogt, Gegenbaur, or Kolliker, 

 and Leuckart expressly states (Z. U., p. G) that he has " sought in vain for the suspensoria 

 described by Milne Edwards in Slephanomia." I observed them, however, very distinctly 

 in Agalma. (See the description of that genus infra.) 



A very peculiar structure is attached to the distal surface of the pneumatocyst of 

 Rldzopltysa (PI. VIII). A great number of elongated, and more or less branched, processes, in 

 fact, project freely from it into the cavity of the pneumatophore. Each process consists of a 

 cellular axis, invested by the ciliated endoderm. The cells of the axis are clear and very 

 large, measuring as much as s^th of an inch in length, and have an opaque, oval "nucleus" 

 jJoth of an inch in diameter, with an oval or circular nucleolus of -sotli of an inch in diameter. 

 Quoy and Gaimard would seem to have originally observed these appendages, and they 

 have since been carefully described by Gegenbaur. Nothing of the kind appears to have 

 been seen in other Phgsophorida, but in Velella and Porpita, the hepatic organ occupies the 

 same position, and hence one is led to suspect a relation between the two structures. It does 

 not appear to me that these ramified processes have any real resemblance to the pneumatic 

 filaments attached to the distal surface of the pneumatocyst in the last-named genera. 



' See Leuckart, Z. U., p. 6, and Z. 3SF. K., p. 67, who states of the pneumatocyst of Apolemia : 

 " Die untere Oeffuuug derselben die in den Reproductionskanal liineinfiihrt." 



' Gegenbaur describes the same structure in Rhizop/iysa, and Leuckart in Apolemia and Forskalia, 

 so that the rule doubtless holds good for all Pliysophoridm. 



^' Aunales des Scieuces Naturelles,' t. xvi, 1841. 



