772 THE ANIMAL KINGDOM. 



vesicle distinctive of Physophoridae, Physalidae, and Discoideae. The float 

 is in the last-named circular and slightly concavo-convex in Porpita, 

 rhomboidal with a diagonal solid vertical crest in Velella ; in both genera 

 composed of a series of concentric chambers which communicate in Velella, 

 have chitinoid walls, and are covered by coenosarc. The chambers open 

 externally on the upper surface by apertures, restricted in number in 

 Velella and placed close to the base of the vertical crest, very numerous in 

 Porpita, urn-like in shape, ranged upon the summits of radial ridges, 

 and becoming closed in the central chambers by the deposition of fresh 

 chitinoid layers. The lower surface of the float in Porpita has hollow 

 radiating ridges from which, and from the central chamber, innumerable 

 pneumatic filaments depend, passing into the walls of the polypite and 

 blastostyles. The corresponding filaments in Velella are few and 

 branched (?). The float of Physalia is large and fusiform, one end long 

 and drawn out, with an aperture into the contained chitinoid saccule. In 

 Physophoridae it is a small more or less globular body. It consists es- 

 sentially of the expanded proximal portion of the coenosarc which has 

 typically, e. g. in Forskalia Ophiura, a medusoid structure. The part 

 corresponding to the manubrium forms an air-vesicle, and what should answer 

 to the cavity of the bell is occupied by a brittle cuticular lamina formed on 

 the surface of the manubrium, which is represented in the species named 

 by two layers of cells. The cuticular structure in shape resembles a retort, 

 mouth downwards. The mouth corresponds to the spot where the manu- 

 brium passes into the wall of the bell, which in this instance contains seven 

 radial canals opening basally into the cavity of the coenosarc. The typical 

 structure is more or less disguised in most cases. The cavity of the air- 

 vesicle is said to open at its apex to the exterior in Rhizophysa a . 



The coenosarc in the Discoideae and Physalia simply invests the 

 float. The zooids in the former are spread over one, the ventral 

 aspect, a large polypite s. gastrozooid in the centre surrounded by 

 a zone of blastostyles, and these in turn by a zone of tentacles or 

 dactylozooids. The coenosarc of Physalia is produced into a prominent 

 crest, vertical and exposed above the water in the natural position. The 

 zooids are aggregated in one or more ventral masses, and the float may in 

 some species attain a length of eight inches. Among Physophoridae the 

 coenosarc of Athorybia is almost globular, of Physophora somewhat elon- 

 gated, but the portion bearing the zooids is short and saccular. In all 

 other Siphonophora it is elongated and tubular. The float of Physo- 

 phoridae, or the nectocalyces of Calycophoridae, occupy its proximal end. 



1 The development of the air-vesicle, which was first observed by Metschnikoff, corresponds, as 

 he stated, with the view that the float is a Medusoid structure. A solid ingrowth of epiblas.t = an 

 entocodon, takes place as in the development of a Medusa where it forms the ectoderm of the sub- 

 umbrella and manubrium. In the case of the float it gives origin to the air-vesicle and its walls. 



