The British genera of fresh-water Polyzoa may be distinguished from one another by means of the following table:—
| 1. |  | Zooecia perfectly distinct from one another. Lophophore circular. Statoblasts absent 2 |
| Colony formed of branching tubes composed of confluent zooecia 3 | ||
| Colony gelatinous, not obviously formed of branching tubes. Lophophore horse-shoe shaped 4 | ||
| 2. |  | Colony consisting of a stolon from which new zooecia originate. These may give rise to new stolons, or directly to new zooecia Victorella |
| Branches composed entirely of club-shaped zooecia, each of which may give off two zooecia near its upper end Paludicella (Fig. 250) | ||
| 3. | | Tubes hyaline or opaque, usually containing numerous oval statoblasts (Fig. 251, B), most of which have a ring of air-cells. Lophophore horse-shoe shaped. |
| ||
| (b) Tubes parallel with one another Alcyonella form of Plumatella (Fig. 246, A) | ||
| Tubes cylindrical, usually dark brown. Statoblasts (Fig. 251, A) few, without air-cells. Lophophore circular Fredericella | ||
| 4. |  | Colony hyaline, usually divided into three or four short lobes. Ectocyst thick. Statoblasts (Fig. 251, C) pointed at each end, with a broad ring of air-cells Lophopus (Fig. 248) |
| Colony slug-shaped, crawling on a flattened sole. Ectocyst rudimentary. Statoblasts (Fig. 251, D) circular, with marginal hooks Cristatella (Fig. 247) | ||
| Colonies consisting of small rosettes, many of which are attached to a thick basal layer of hyaline ectocyst. Statoblasts circular, with marginal hooks. (Not recorded as British) Pectinatella | ||
Reproductive Processes of Polyzoa in general.
In studying the reproductive processes of Polyzoa, we have to deal with two very distinct phenomena; firstly, with the development of eggs; and secondly, with the formation of buds.
The process of budding usually does no more than increase the number of individuals in a colony which already exists, and is seldom responsible for the commencement of a new colony. In Loxosoma, however, the buds break off and lead an independent existence; and in the Phylactolaemata a large proportion of the colonies have their origin in the statoblasts. In certain cases, again, new colonies may be formed by the detachment of parts of an old one, as by the fission of Cristatella and Lophopus, or by the breaking up of a richly-branched species into several colonies by the decay of the proximal parts.
We may then in the majority of cases look to an embryo for the foundation of a new colony. The embryo develops into a larva, which, after a period in which it swims freely, settles down, and is metamorphosed into the first zooecium. This primary individual forms the starting-point of a colony, and often differs to a considerable extent from the other zooecia which arise from it. In Cyclostomata, for instance, the proximal end of the primary zooecium permanently retains the disc-like shape assumed by the young larva when it first fixed itself. The primary zooecium may be recognised with equal ease in many Cheilostomata, and may differ from its successors by possessing a richer development of marginal spines, or in other respects.
Reproductive Organs.—Eggs and spermatozoa are commonly found in the same colony, either in different individuals, or else in the same zooecium (see Fig. 234, p. [469]). In some cases, the zooecium first develops spermatozoa, and later eggs. The Entoprocta have a more marked separation of the sexes than obtains in other Polyzoa. The genus Loxosoma is perhaps always dioecious (i.e. with separate sexes). Pedicellina is sometimes found with ovaries and testes in the same individual, sometimes with these organs in different individuals; and it is not clear whether a given species always behaves alike in these respects.
The reproductive organs of the Entoprocta open by ducts of their own into the vestibule. In the Ectoprocta they are developed in the body-cavity, and they have no ducts.
The fate of the ripe egg differs widely in different cases. In the Entoprocta it develops in a kind of brood-pouch formed from part of the vestibule. The fact that in Pedicellina (Fig. 243) the embryos grow largely during their development, shows that nutritive material must be supplied to them from the parent. There is reason to believe that the epithelium of the brood-pouch is responsible for this process. The eggs are also known to develop at the expense of nutritive substances prepared by the parent in the ovicells of the Cyclostomata. In other cases, as in some species of Alcyonidium, the egg is large, and its copious yolk doubtless supplies a large part of the material required for development.
In the Ectoprocta, development takes place in a variety of places. In most Cheilostomata a single egg passes into the globular ovicell, which is formed above the orifice of many of the zooecia. In certain Ctenostomata,[[569]] Phylactolaemata,[[570]] and Cyclostomata,[[571]] the ripe egg is taken up by a rudimentary polypide-bud, which is specially formed for the purpose. In the Ctenostomata and in the fresh-water Polyzoa these buds, if present, are found in ordinary zooecia which do not become modified externally in any special way. In the Cyclostomata (Crisia), on the contrary, the formation of the polypide-bud is intimately bound up with the development of the ovicell. The number of the zooecia which produce eggs that are capable of development is greatly restricted in this group. The ovicell, which contains numerous embryos, is not merely a portion of a zooecium, as in the Cheilostomata; but it is probably to be regarded as a modification of the entire fertile zooecium or zooecia. These take on an appearance widely differing from that of the ordinary zooecia, and in course of time give rise to the ovicells (see Fig. 237).