Apoleminae.—Similar to the above, but without tentilla. Apolemia—this genus attains a length of two or three metres. Mediterranean Sea. Dicymba, Indian Ocean.
Physophorinae.—The pneumatophore larger in proportion than it is in the preceding families. The stolon is short, and bears rows of nectocalyces at the upper end. The gastrozooids, dactylozooids, and gonozooids are arranged in verticils on the lower expanded part of the stolon. Hydrophyllia absent. Physophora, cosmopolitan in the areas of warm sea water.
Fam. 2. Auronectidae.—The pneumatophore is large. The stolon is reduced to a spongy mass of tissue on the under side of the pneumatophore, and this bears numerous cormidia arranged in a helicoid spiral. Projecting from the base of the pneumatophore there is a peculiar organ called the "aurophore," provided with an apical pore. This organ has been described as a specially modified nectocalyx, but it is probably a specialised development of the epithelium-lined portion of the pneumatophore of other Physophorae. The Auronectidae are found only at considerable depths, 300 to 1400 fathoms, and are probably specially adapted to that habitat. Rhodalia, Stephalia, Atlantic Ocean.
Fam. 3. Rhizophysaliidae.—The pneumatophore is large, or very large, in this family. The zooids are arranged in horizontal rows on the under side of the pneumatophore (Physalia), or in a helicoid spiral on a short stolon (Epibulia). There are no nectocalyces nor hydrophyllia.
The genus Physalia is the notorious "Portuguese Man-of-War." The pneumatophore is a large bladder-like vesicle, sometimes attaining a length of 12 cm. One species described by Haeckel under the generic name Caravella has a pneumatophore 30 cm. and more in length, and dactylozooids attaining a length of 20 metres. It is a curious fact that only the male colonies of Physalia are known, and it is suggested that the female may have quite a different form.[[340]] Epibulia has a much smaller bladder than Physalia. Both genera have a cosmopolitan distribution at the surface of the warm seas.
Fam. 4. Chondrophoridae.—This family stands quite by itself in the sub-order Physophorae, and is placed in a separate division of the sub-order by Chun, who gives it the name Tracheophysa. The essential distinguishing characters of the family are the large polythalamic pneumatophore and the single large central gastrozooid.
The colony is disc-shaped, and has a superficial resemblance to a Medusa. On the upper side is the flattened pneumatophore, covered by a fold of tissue continuous with that at the edge of the disc. In Velella a vertical triangular sail or crest rises from the upper side, but this is absent in Porpita.
The mouth of the gastrozooid opens into a large digestive cavity, and between this and the under surface of the pneumatophore there is a glandular spongy tissue called the liver. The liver extends over the whole of the under side of the pneumatophore, and sends processes round the edge of the disc into the tissues of its upper surface. Intimately associated with the liver, and penetrating its interstices, is an organ which appears to be entirely composed of nematocysts, derived from the ectoderm, and called the central organ. At the margin of the disc there is a fringe of simple digitiform dactylozooids, and between the dactylozooids and the centrally placed gastrozooid are numerous gonozooids. Each of the gonozooids is provided with a distinct mouth, and bears the gonophores, which escape before the ripening of the gonads as the free-swimming Medusae called Chrysomitra. The pneumatophore consists of a number of annular chambers arranged in a concentric manner round the central original chamber formed from a modified zooid. These annular chambers are in communication with one another, and have each two pores (pneumatopyles) opening above to the exterior. The most remarkable feature, however, of the system is a series of fine branching tubes ("tracheae"), which pass from the annular chambers of the pneumatophore downwards into the hepatic mass and ramify there.
There are two well-known genera: Velella with a sail, and Porpita without a sail. They are both found at the surface of the warmer regions of the great oceans and in the Mediterranean. Velella sometimes drifts on to British coasts from the Atlantic.
The genus Discalia has a much more simple octoradial structure. It was found at depths of 2600 and 2750 fathoms in the Pacific Ocean.