Page:EB1911 - Volume 22.djvu/57

Rh described from Palaeozoic strata. They constitute a small proportion of the recent Polyzoa. The Cheilostomata are usually believed to have made their appearance in the Jurassic period. They are the dominant group at the present day, and are represented by a large number of genera and species. The Phylactolaemata are a small group confined to fresh water, and possess clear indications of adaptation to that habitat. The fresh-water fauna also contains a representative of the Entoprocta (Urnatella), two or three Ctenostomes, such as Victorella and Paludicella, and one or two species of Cheilostomata. With these exceptions, the existing Polyzoa are marine forms, occurring from between tide-marks to abyssal depths in the ocean.

The Polyzoa are colonial animals, the colony (zoarium) originating in most cases from a free-swimming larva, which attaches itself to some solid object and becomes metamorphosed into the primary individual, or “ancestrula.” In the Phylactolaemata, however, a new colony may originate not only from a larva, but also from a peculiar form of bud known as a statoblast, or by the fission of a fully-developed colony. The ancestrula inaugurates a process of budding, continued by its progeny, and thus gives rise to the mature colony. In Loxosoma the buds break off as soon as they become mature, and a colonial form is thus hardly assumed. In other Entoprocta the buds retain a high degree of individuality, a thread-like stolon giving off the cylindrical stalks, each of which dilates at its end into the body of a zooid. In some of the Ctenostomata the colony is similarly constituted, a branched stolon giving off the zooids, which are not connected with one another. In the majority of Ectoprocta there is no stolon, the zooids growing out of one another and being usually apposed so as to form continuous sheets or branches. In the encrusting type, which is found in a large proportion of the genera, the zooids are usually in a single layer, with their orifices facing away from the substratum; but in certain species the colony becomes multilaminar by the continued superposition of new zooids over the free surfaces of the older ones, whose orifices they naturally occlude. The zoarium may rise up into erect growths composed of a single layer of zooids, the orifices of which are all on one surface, or of two layers of zooids placed back to back, with the orifices on both sides of the fronds or plates. The rigid Cheilostomes which have this habit were formerly placed in the genus Eschara, but the bilaminar type is common to a number of genera, and there can he no doubt that it is not in itself an indication of affinity. The body-wall is extensively calcified in the Cyclostomata and in most Cheilostomata, which may form elegant network-like colonies, as in the unilaminar genus Retepora, or may consist of wavy anastomosing plates, as in the bilaminar Lepralia foliacea of the British coasts, specimens of which may have a diameter of many inches. In other Cheilostomes the amount of calcification may be much less, the supporting skeleton being largely composed of the organic material chitin. In Flustra and other forms belonging to this type, the zoarium is accordingly flexible, and either bilaminar or unilaminar. In many calcareous forms, both Cheilostomes and Cyclostomes, the zoarium is rendered flexible by the interposition of chitinous joints at intervals. This habit is characteristic of the genera Crisia, Cellaria, Catenicella and others, while it occurs in certain species of other genera. The form of the colony may thus be a good generic character, or, on the contrary, a single genus or even species may assume a variety of different forms. While nearly all Polyzoa are permanently fixed to one spot, the colonies of Cristatella and Lophopus among the Phylactolaemata can crawl slowly from place to place.

Anatomy.—The zooids of which the colonies of Ectoprocta are composed consist of two parts: the body-wall and the visceral mass (figs. 6, 9). These were at one time believed to represent two individuals of different kinds, together constituting a zooid. The visceral mass was accordingly, termed the “polypide” and the body-wall which contains it the “zooecium.” This view depended principally on the fact that the life of the polypide and of the zooecium are not coextensive. It is one of the most remarkable facts in the natural history of the Polyzoa that a single zooecium may be tenanted by several polypides, which successively degenerate. The periodical histolysis may be partly due to the absence of specific excretory organs and to the accumulation of pigmented excretory substances in the wall of the alimentary canal. On the degeneration of the polypide, its nutritive material is apparently absorbed for the benefit of the zooid, while the pigmented substances assume a spheroidal form, which either remains as an inert “brown body” in the body-cavity or is discharged to the exterior by the alimentary canal of the new polypide. This is formed as a two-layered “polypide-bud,” which usually develops from the inner side of the zooecial wall, and soon occupies the place of the previous polypide. The inner layer of the polypide-bud gives rise to the structures usually regarded as ectodermic and endodermic, the outer layer to the mesodermic organs.

The polypide consists of a “lophophore” bearing a series of ciliated tentacles by which Diatoms and other microscopic bodies are collected as food, of a U-shaped alimentary canal, and of a central nervous system. While the mouth is invariably encircled by the bases of the tentacles, the anus lies within the series in the Entoprocta and outside it in the Ectoprocta. The lophophore is a simple circle in all Polyzoa except in the Phylactolaemata, where it typically has the form of a horse shoe outlined by the bases of the tentacles. In Fredericella belonging to this order it is, however, circular, but the systematic position of the genus is sufficiently indicated by its possession of an “epistome,” a lip-like structure guarding the anal side of the mouth in all Phylactolaemata and absent throughout the Gymnolaemata. The cavities of the hollow tentacles open into a circular canal which surrounds the oesophagus at the base of the lophophore. This is continuous with the general body-cavity in the Phylactolaemata, while in the Gymnolaemata it develops in the bud as a part of the body-cavity, from which it becomes completely separated. In the Entoprocta the tentacles are withdrawn by being infolded into the “vestibule,” a depression of the oral surface which can be closed by a sphincter muscle. In the Ectoprocta they are retractile into an introvert, the “tentacle-sheath” (fig. 9), the external opening of which is the “orifice” of the zooecium. In the Cyclostomata, further distinguished by the cylindrical or prismatic form of their highly calcified zooecia, the orifice is typically circular, without any definite closing organ. In the Cheilostomata it is closed by a chitinous (rarely calcareous) “operculum” (fig. 9, C), while in the Ctenostomata it is guarded by a delicate membrane similar to a piece of paper rolled into a longitudinally creased cylinder. During retraction this “collar” lies concealed in the beginning of the introvert. It becomes visible when the polypide begins to protrude its tentacles, making its appearance through the orifice as a delicate hyaline frill through which the tentacles are pushed.

In the Phylactolaemata the outermost layer of the body-wall is a flexible uncalcified cuticle or “ectocyst,” beneath which follow in succession the ectoderm, the muscular layers and the coelomic epithelium. In a few Gymnolaemata the ectocyst is merely chitinous, although in most cases the four vertical walls and the basal wall of the zooecium are calcareous. The free (frontal) wall may remain membranous and uncalcified, as in Membranipora (figs. 8 A, 9 A), but in many Cheilostomes the frontal surface is protected by a calcareous shield, which grows from near the free edges of the vertical walls and commonly increases in thickness as the zooecium grows older by the activity of the “epitheca,” a layer of living tissue outside it. The body-wall is greatly simplified in the Gymnolaemata, in correlation with the functional importance of the skeletal part of the wall. Even the ectoderm can rarely be recognized as an obvious epithelium except in regions where budding is taking place, while muscular layers are always absent and a coelomic epithelium can seldom be observed. The body-cavity is, however, traversed by muscles, and by strands of mesodermic “funicular tissue,” usually irregular, but sometimes constituting definite funiculi (fig. 6, x, x′). This tissue is continuous from zooecium to zooecium