They contain a central endodermic canal from which, as in Crystallodes (Agalma) rigidum, a process may extend to the lateral angles. (6) Necto-calyces absent in Discoideae, Physalia, Athorybia, and Rhizophysa. They resemble a craspedote Medusa with four radial canals, minus a manubrium, mouth and tentacles4. (7) The pneumatophore ox float, an airvesicle distinctive of Physophoridae, Physalidae, and Diseoideae. 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 essentially 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 manubrium 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 Rhizophysa1.

1See p. 775, post, on the character of the colony.

2They are due to the contraction of endodermal circular muscles.

3 An opening is present at the apex of the hydrocyst of Halistemma tergestinum (pictum) and of Physalia (?). So too in a young Agalmopsis (Agalma) Sarsii, where it afterwards closes. The apex of the hydrocyst is pigmented, and it has been observed that irritation of the animal in Forskalia causes this pigment to be shed into the water, giving the latter an opaque and red tint. The contained fluid is highly albuminous and dense, and probably serves to keep the walls of the hydrocyst tense, and thus render it more sensitive (Korotneff). Hydrocysts occur among the nectocalyces in Apolemia but not in other Siphonophora.

1But see Metschnikoff, • Medusologische Mittheilungen,' Arb. Zool. Inst. Wien, vi. p. 238 (p. 2 of paper). The female, according to him, only brings one ovum to maturity.

2This term is used in the text to denote the sub-order, not the family.

3Chun's genus Lilyopsis is no exception to this statement, for the groups of zooids are not detached; see note, p. 773, and SB. Akad. Berlin, 1886, p. 688.

4 The absence of a manubrium is a great peculiarity; it has been observed by Mereschkowsky as a constant occurrence in certain species of Bougainvillea. See A. N. H. (5), iii. 1879.

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 elongated, but the portion bearing the zooids is short and saccular. In all other Siphonophora it is elongated and tubular. The float of Physophoridae, or the nectocalyces of Calycophoridae, occupy its proximal end.

1The 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 epiblast = 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.

The nectocalyces of the Physophoridae, when present, come next to the float. The rest of the coenosarc carries the other zooids which are arranged in distinct groups in the Physophorid Apolemia, and in all Caly-cophoridae. Segments bearing several groups of zooids are detached in Apolemia, and single groups (bract, polypite, tentacle) are set free in some species of Diphyes, in Abyla, Monophyes, and Muggiaea. These single groups are known as Diphyozooids, and have received special names, e.g. Diplophysa (Monophyes), Endoxia, etc. (Diphyes). The Diphyozooid developes a succession of sexual zooids. The nectocalyces and zooids are developed along one and the same aspect of the coenosarcal tube; but, owing to the fact that it is spirally twisted in Physophoridae, they appear to be disposed in two or more rows, and the real arrangement is not evident. The twist of the nectocalycine region is in the opposite direction to that of the rest of the coenosarc. New nectocalyces in the Physophoridae appear distally to the float: the other zooids distally to the nectocalyces both in Calycophoridae and Physophoridae1. The coenosarcal tube is eminently contractile.