This second order of the Oceanic Hydrozoa comprises those Siphonophora in which the hydrosoma consists of several polypites united by a flexible, contractile, unbran3ched or very slightly branched coenosarc, the proximal extremity of which is modified into a "pneumatophore" and is sometimes provided with "nectocalyces." The polypites have either a single basal., tentacle, or the tentacles arise directly from the coenosarc. "Hydrophyllia" are commonly present. The reproductive bodies are developed upon gonoblastidia.

The coenosarc in the Physophoridae, like that of the Calycophoridae, is perfectly flexible and contractile; but it is not necessarily elongated, being sometimes spheroidal or discoidal. The proximal end of the coenosarc" expands into a variously-shaped enlargement, whose walls consist of both ectoderm and endoderm, and which encloses a wide cavity in free communication with that of the coenosarc, and, like it, full of the nutritive fluid. From the distal end, or apex, of this cavity depends a sac, variously shaped, but always with tough, strong, and elastic walls, composed of a substance which is stated to be similar to chitine in composition, and more or less completely filled with air" (Huxley). The large proximal dilatation of the coenosarc is termed the "pneumatophore," whilst the chiti-nous air-sac which it contains is termed the "pneumatocyst" (fig. 54, 1). The pneumatocyst is held in position by the reflection of the endoderm of the pneumatophore over it, and it doubtless acts as a buoy or "float." In the Portuguese calyx; h Hydroecium ; c Coenosarc, carrying polypites each with its bract and tentacle.

Fig. 55.   Calycophor idae. Diphyes appendi culata (after Kolliker). v Proximal nectoca lyx; v' Distal nectoman of war (Physalia) the pneumatocyst communicates with the exterior by means of an aperture in the ectoderm of the pneumatophore. In Velella and Porpita the pneumatocyst communicates with the exterior by means of several similar openings called

Fig. 55. - Calycophor-idae. Diphyes appendi-culata (after Kolliker). v Proximal nectoca-lyx; v' Distal nectoman-of-war (Physalia) the pneumatocyst communicates with the exterior by means of an aperture in the ectoderm of the pneumatophore. In Velella and Porpita the pneumatocyst communicates with the exterior by means of several similar openings called "stigmata;" and from its distal surface depend numerous slender processes containing air, and known as "pneumatic filaments."

The polypites of the Physophoridae resemble those of the Calycophoridae in shape, but the tentacles have a much more complicated structure, and are sometimes many inches in length, as in Physalia. The "hydrophyllia" have essentially the same structure as those of the former order. There occur also in the Physophoridae certain peculiar bodies, termed "hydrocysts" or "feelers" ("fuhler" and "taster" of the Germans). These resemble immature polypites in shape, consisting of a prolongation of both ectoderm and endoderm, usually with a tentacle, and containing a diverticulum of the somatic cavity, the distal extremity being closed, and furnished with numerous large thread-cells. They are looked upon as "organs of prehension and touch," and they are somewhat analogous to the "nematophores" of the Plumularians.

As regards the reproductive organs, they are developed upon special processes or "gonoblastidia," and they may remain permanently attached, or they may be thrown off as free-swimming medusoids. In many of the Physophoridae the male and female gonophores differ from one another in form and size, and they are then termed respectively "andro-phores" and "gynophores." As regards their development, the Physophoridae obey the same general law as the Calycophoridae.

In Physophora the hydrosoma consists of a filiform coenosarc, which bears the polypites and their appendages, and dilates proximally into a pneumatophore. Below this point the coenosarc bears a double row of nectocalyces, which are channelled on their inner faces to allow of their attachment to the coenosarc. There are no hydrophyllia, but there is a series of "hydrocysts" on the proximal side of the polypites.

Physalia, or the Portuguese man-of-war (fig. 56, a) is composed of a large, bladder-like, fusiform "float" or pneumatophore-sometimes from eight to nine inches in length - upon the under surface of which are arranged a number of polypites, together with highly contractile tentacles of great length, "hydrocysts," and reproductive organs. Physalia is of common occurrence, floating at the surface of tropical seas; and fleets of it are not uncommonly driven upon our own shores.

In Velella (fig. 56, b) the hydrosoma consists of a widely-expanded pneumatophore of a rhomboidal shape, carrying upon its upper surface a diagonal vertical crest. Both the horizontal disc and the vertical crest are composed of a soft marginal "limb," and a central more consistent "firm part." "To the distal surface of the firm part of the disc are attached the several appendages, including - 1, a single large polypite, nearly central in position ; 2, numerous small gonoblastidia, which resemble polypites, and are termed 'phyogemmaria;' and, 3, the reproductive bodies to which these last give rise. The tentacles are attached, quite independently of the polypites, in a single series along the line where the firm part and limb of the disc unite. There are no hydrocysts, nectocalyces, or hydrophyllia. On all sides the limb is traversed by an anastomosing system of canals, which are ciliated, and communicate with the cavities of the phyogemmaria and large central polypite" (Greene). Velella is about two inches in length by one and a half in height. It is of a beautiful blue colour and semi-transparent, and it floats at the surface of the sea, with its vertical crest exposed to the wind as a sail.

Fig. 56.   Physophoridae. a Portuguese man   of  war (Physalia utriculus), showing the fusiform float and the polypites and tentacles (after Huxley); b Velella vulgaris (after Gosse).

Fig. 56. - Physophoridae. a Portuguese man - of- war (Physalia utriculus), showing the fusiform float and the polypites and tentacles (after Huxley); b Velella vulgaris (after Gosse).