MORPHOLOGY. 7 



In form, the pneumatocyst'(and the pneumatophore which contains it) varies from that 

 of a spheroid {Bhizophjsa, AthoryUd) or of an elongated oval {Physophora) to that of a 

 cylinder with a narrow inferior neck {Ayalma), or an irregular, pear-shape {Plii/salia), or the 

 figure of a flattened disc {Velella, Porpita). No less docs one genus differ from another 

 in the proportional size of this apparatus. In Phijsalia, Velella, and Porpita, it occupies 

 almost the whole of the hydrosoma, and constitutes the largest and most conspicuous 

 part of the body, while in such genera as Ayalma and Forskalia it attains to only such 

 insignificant proportions as to be readily overlooked. 



Its functional importance, of course, depends very nearly on its relative size. When 

 it is large it must necessarily play a very considerable part in determining the habits of 

 its possessor, though we have at present no information as to whether it acts as a permanent 

 buoy or whether it can be voluntarily emptied and filled again.^ When the pneumatic 

 apparatus is so small in proportion to the rest of the organism, as in many Physophorida, 

 on the other hand, its office in the economy ceases to be easily comprehensible, and can 

 hardly be very important. 



The last modification of the proximal end of the hydrosoma of which I have to speak 

 is an organ — I mean the disc or umbrella of LucernariadcE — which is commonly confounded 

 under one head, with others — the nectocalyces — of quite distinct structure ; apparently, 

 because, like the latter, it acts as an organ of propulsion. Nevertheless, in development, 

 in structure, and in even in mode of action, the umbrella is altogether distinct from any other 

 organ possessed by the Hijdrozoa. 



I am inclined to think that the rudiments of this structure are visible in the Ilydrida, 

 and in some Corynidm and Sertidariada. In examining a Hydra carefully, it is seen that 

 tlie tentacles do not immediately surround the oral aperture ; but they arise in a circle 

 at some distance below it, so that the oral aperture is supported on a kind of conical 

 crater, from whose base they take their origin. 



In some Campaimlariada , this separation of the polypite into a distal and a proximal 

 portion is still more marked. The latter ends distally in a truncated disc, from whose edoes 

 the tentacula spring, wiiile the oral division of the polypite is longer than the basal, and 

 arise from the truncated face of the latter by a narrow neck. The like structure is 

 observable in the " Hydra tuba," the larval form of the Lucernarian Medusa, and from 

 hence to the structure of Liicernaria, there is but a step. In the latter genus, in fact, the 

 discoid, proximal portion of the polypite, or proximal end of the hydrosoma, is greatly 

 enlarged, and produced into eight obtuse lobes, each of which gives rise to a few short 

 tentacles ; while the short distal division of the animal lies in the middle of tlie disc. 



In Tuhdaria, the buds which give rise to the reproductive organs are developed from 

 the surface of the polypite within the margins of the proximal disc and internal to its 



^ There is very little good evidence to be met with on this much disputed question. I have seen 

 part of the contained air to all appearance voluntarily expelled from the pneumatocyst of Rhizophysa, 

 and Forskal appears to have witnessed something of the same kind, for he says of his Phtjsophora 

 filiformis : " VivEe, vesica acre plena, taraen subsidere possunt ; dum corpus arctaudo, se reddunt specifice 

 graviores." Eschscholz found, on irritating a young P/n/salia five lines long, that it " suddenly expelled 

 all the air from the bladder, and sank to the bottom of the glass" (p. 159). 



