There are three forms of larva among Gymnolaemata, the bivalved larva of Flustrella and of Membranipora, the latter known as Cyphonautes: the larva of other Cheilostomata, and Ctenostomata, and that of Cyclostomata. The Cheilostomatous and Ctenostomatous larva has an aboral face separated by a ciliated ring or corona from an oral face. In both there is an aboral calotte or retractile disc, surrounded by, or bearing sense-hairs, sometimes regarded as the homologue of the foot-gland of the Entoprocta. On the oral face are two structures: one the 'anterior ectodermal furrow' of Vigelius,'oral furrow' of Nitsche and Claparede, the 'fente' of Barrois, the other, the 'sucker' of Vigelius, 'stomach' of Barrois, both produced by invaginations of the ectoderm. The sucker appears to be the organ by which the larva ultimately fixes itself; and it might, therefore, be considered, at least functionally, as the homologue of the foot-gland of Loxosoma. The larva of Cyclostomata is ciliated and barrel-shaped. At one end is the mouth leading into a stomach.
At the other is a prominence, the homologue of the ciliated disc inclosed in a cylindrical sheath, the homologue of the corona (?). When a larva fixes itself, its organs, whatever they are, appear to undergo atrophy, and the tentacles, digestive tract, muscles, and nervous system of the adult zooid, make their appearance. The Phylactolaemata also reproduce by statoblasts or winter buds formed from the funiculus. The cells at one pole of the bud grow round the remaining cells, and form at their inner ends the chitinous investment and chitinoid marginal air-cells of the fully formed statoblast. The statoblasts are discharged on the death and decay of the parent, and the whole colony in Pectinatella is at this time set free, and floating along the stream scatters them over a wide area. The same is true of Cristatella. The statoblast remains quiescent through the winter season. In spring it gives origin to a small non-ciliated, but fully formed individual, which fixes itself, and produces a colony by gemmation.
All Polyzoa increase by budding, and the bud appears to be formed by tissue elements derived from the corresponding elements of the parent (see p. 236). In all the marine Polyzoa the tentacles, digestive tract, and its retractor muscles, with the nervous system of the individual are lost in the older parts of the colony. They degenerate into the 'brown body.' A new set of organs is produced from the endocyst by budding, and the brown body appears to pass into the new stomach where it may break down and perhaps be partially digested. It or its remnants are eventually expelled.
The Phylactolaemata are confined to fresh-water. The Gymnolaemata with the exception of Paludicella are marine, and are most plentiful near the shore, and at moderate depths. Certain Cheilostomata were, however, dredged at great depths in the voyage of H.M.S. Challenger, e.g. in the North Atlantic at 2750 fathoms, in the North Pacific at 3125. The Cyclostomata and Cheilostomata occur fossil. The first-named appear in Cambrian and Silurian strata, the second are sparingly represented in the older strata, but become numerous in the Upper Greensand, whilst the Cyclostomata diminish in numbers from Tertiary strata to the present time. Of living genera Stomatopora among Cyclostomata^ and Hippothoa among Cheilostomata are said to appear in Silurian times. Many Jurassic genera still survive.
The class Polyzoa is subdivisible as follows:
Lophophore praeoral; mouth and anus within the lophophoral area; no coelome; a pair of nephridia \ hermaphrodite. Zoxosoma, non-colonial; Pedicellina, colonial. See note I, p. 705.
Lophophore post-oral; anus external to it; a large coelome; sexes sometimes separate (?). Colonial.
Lophophore horse-shoe shaped; a moveable epistome overhanging the mouth; fresh-water; Cristatella, Pectinatella, Zophopus, Alcyonella, Plumatella, Fredericella.
Lophophore orbicular; no epistome; marine with the exception of Paludicella.
'Cell mouth,' i.e. aperture left when tentacle-sheath is retracted, not guarded by an operculum or processes; zooecia tubular; most genera are fossil; e. g. Crista, Diastppora, etc.
'Cell-mouth ' guarded by seta-like processes; zooecium never calcareous; stem and root-cells often present; e.g. Alcyonidium, Vesicularia (= Valkeria), Paludicella, etc.
'Cell-mouth,' guarded by a thickened and moveable operculum; oral area to a great extent membranous; ova usually matured in ooecia; avicularia and vibracula often present; e.g. Aetea, Scrupocellaria, Flustra, Membranipora, Zepralia, Eschara, Cellepora, etc.
Entoprocta. Zoxosoma, Harmer, Q. J. M. xxv. 1885, with lit. cited; Pedicellina, Nitsche, Z. W. Z. xx. 1870 , its life history, Harmer, Q. J. M. xxvii. (2), 1886; Barrois, A. Sc. N:, (7), i. 1886.
Ectoprocta, see p. 238. Fresh-water Polyzoa, Jullien, Bull. Soc. Zool. de France, x. 1885; Polyzoa (Cheilostomata), Busk, Challenger Reports, x. 1884.
Fossil Polyzoa, Zittel, Handbuch der Palaeontologie, Abth. i, Palaeozologie, i. 1880, p. 575.
Ectodenn, etc, Ostroumoff, Z. A. viii. 1885.
Intertentacular organ, Farre, Ph. Tr. 127, 1837, pp. 408, 412; Hincks, A. N. H. (2), viii. 1851, p. 355; Id. Brit. Marine Polyzoa, i. p. lxxxix.
Special aperture for escape of ova; in Farrella (=Zaguncula), P. J. van Beneden, Mem. Ac. Roy. Belg. xviii. 1845, p. 18; in Hypophorella, Ehlers, Abhandl. K. Ges. der Wissenschaften, Gottingen, xxi. 1876, p. 66 (not seen).
Rudimentary tentacle sheath, etc.,for escape of ova; in Alcyonella, Metschnikoff, Bull. Ac. Imp. St. Petersburg, xv. 1871, p. 507; Nitsche, Z. W. Z. xxii. 1872, p. 467; in Valkeria, etc. Joliet, A. Z. Expt. vi. 1877, p. 262.
Floating colonies of Pectinatella, Kraepelin, Z. A. vii. 1884.
Embryology of Phylactolaemata, Reinhard, Z. A. iii. 1880; cf. on Metamorphosis of do., Ostroumoff, Z. A. ix. 1886; of Gymnolaemata, Barrois, Journal de l'Anat. et Physiol, xviii. 1882; cf. A. N. H. (5), x. 1882; Id. A. Sc. N., i. 1886; Vigelius, Mitth. Zool. Stat. Naples, vi. 1886; of Cyclostomata, Ostroumoff, Z. A. ix. 1886.
Ooecium of Bugula, Vigelius, op. cit. p. 512.
Gonoecium and Gonocyst, Hincks, 'Marine Polyzoa,' i. p. xcvi; p. 418.