Regeneration of the Polypide.—The duration of life of the polypide does not correspond with that of the zooecium, which has a succession of polypides. This curious fact is probably the result of the absence of definite excretory organs, the function of which seems to be performed largely by the stomach. The wall of this organ becomes charged with brown granules (probably excretory), and after a time the entire polypide degenerates, de creasing in size and ultimately becoming a small, rounded "brown body" (fig. 8), which owes its colour to these granules. The substances of nutritive value have probably been absorbed, for future use, by the cells which surround the degenerating poly pide. An internal polypide-bud is simultaneously developed, and in some species its stomach en velopes the brown body, which passes into its cavity and is rejected with the faeces. In other cases the brown body remains as an inert mass in the zooecium, and the occurrence of several brown bodies indicates a corresponding number of degenerated polypides.
Polymorphism.—In the majority of Cheilostomata, certain zooecia have merely a vestige of a polypide, and the operculum, now known as the "mandible," and its muscles become modified for prehension. These units are known as "avicularia," from their resemblance to the head of a bird, in Bugula (fig. 9) and other genera in which they attain their highest development. The avic ularia may be "vicarious," in series with the zooecia, which they may surpass in size, the mandibles being often much larger than the opercula (fig. 1o) ; or they are "adventitious" (fig. 5), when they occur as appendages of ordinary zooecia. They are either "sessile," closely attached to the zooecia (fig. 5) or stalked (fig. 9), and they show a wide range of form, in different species. The "vibracula" (fig. 1), which are found in a small number of Cheilostomata, have the operculum transformed into a long "seta," which sweeps through the water. In Caberea the vibracula of a branch have been observed to move in unison, but this is exceptional. The avicularia and vibracula appear to be defensive organs, and they doubtless ward off the attacks of some predacious animals. They probably pre vent the overgrowth of the colony by en crusting organisms, by discouraging the attachment of larvae, and they may also serve to keep the colony clean, by dis lodging foreign particles which might otherwise settle on it.
Reproduction.—The reproductive organs occur in the body cavity in Ectoprocta ; and organs of both sexes may be produced by a single zooecium, simultaneously or successively. The colony seems to be gen
erally bisexual, even when testes and ovaries are found in different zooecia. The ciliated larvae, in this group, rarely possess an alimentary canal. If this is present they are known as Cyphonautes, a common con stituent of the floating fauna, especially of coastal waters. If it is absent the tissues are charged with nutritive yolk. In either case, the larva attaches itself, loses its lar val organs and becomes the "ancestrula" or first zooecium of the colony, developing a polypide as an internal bud. The ances trula buds off other zooecia, which repeat the process, thus building up the colony; and the zooecia develop their polypides in the same way as the ancestrula. In species with a Cyphonautes the egg develops in the water, but in most cases it develops in the parent colony. The characteristic "ovicells" of Cheilostomata (fig. 5) are external brood-spaces into which the egg passes when it is laid, and are formed in part by the distal end of the fertile zooecium, but principally by the frontal surface of the succeeding zooecium. The egg passes from the body-cavity to the exterior (in the few cases where the process has been observed) through the "intertentacular organ," a ciliated tube between the bases of the two tentacles nearest the anus, or through a pore found in the same position. The Cheilostome ovi cell nearly always contains a single egg, but in the Cyclostomata the ovicell (fig. 3) contains very numerous embryos, which have been produced by the fission of a primary embryo, developed from an egg. In Phylactolaemata the polypide is pro duced by the larva while it is still free, several polypides occurring in Cristatella before the larva attaches itself. Another form of reproduction is found in this group, where the zooecium produces internal buds from the "funi culus," a cord connecting the blind end of the stomach with the body-wall. These special buds are known as "statoblasts" (fig.
12), and each is protected by a strong chitinous shell, the outer part ("annulus") of which is modified as a ring containing air cells which enable the statoblast to float on the surface of the water when it becomes free. In temperate latitudes this happens in the late summer or autumn, and the statoblast develops into a new colony in the ensuing spring.