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 {309}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_.