Therein an order Thoracica comprehends the pedunculate Lepadidae, together with the operculate and sessile Balanidae and Verrucidae; a single species without cirrhi constitutes the order Apoda, and a single species with only three pairs of cirrhi the order Abdominalia.
In Leptolinae the actinula becomes the sessile polyp which has acquired the power of budding and producing individuals either of its own or of a higher rank; it represents a persistent larval stage and remains in a sexually immature condition as a neutral individual, sex being an attribute only of the final stage in the development, namely the medusa.
The barnacles and their allies, forming the group Cirripedia or Thyrostraca, sometimes treated as a separate sub-class, are distinguished by being sessile in the adult state, the larval antennules serving as organs of attachment, and the antennae being lost.
The anther is developed before the filament, and is always sessile in the first instance, and sometimes continues so.
The vernacular name barnacle, traceable to the fable of pedunculate cirripedes hatching out into bernicle geese, has also been transferred to the sessile cirripedes, which are popularly known as acorn barnacles.
Trophosome as in Eucopidae; gonosome, sessile gonophores.
The most usual condition, however, is that in which sessile medusoid gonophores or sporosacs are produced.
The polyp is regarded, on this view, as a form phylogenetically older than the medusa, in short, as nothing more than a sessile actinula.
A conical, thorn-like projection from the base of the pore, sometimes found also in dactylopores; sessile gonophores.
The bathymetric range of sessile as well as pedunculate forms down to such depths as twelve or eighteen thousand feet - Verruca quadrangularis, Hoek, 1900 fathoms; Sealpellum regium, Wyville Thomson, 2850 FIG.