Page:EB1911 - Volume 22.djvu/58

Rh through perforated “rosette-plates” in the dividing walls. In the Phylactolaemata a single definite funiculus passes from the body-wall to the apex of the stomach. This latter organ is pigmented in all Polyzoa, and is produced, in the Ectoprocta, beyond the point where the intestine leaves it into a conspicuous caecum (fig. 6, v). The nervous system is represented by a ganglion situated between the mouth and the anus. The ovary (o) and the testis (t) of Ectoprocta are developed on the body-wall, on the stomach, or on the funiculus. Both kinds of reproductive organs may occur in a single zooecium, and the reproductive elements pass when ripe into the body-cavity. Their mode of escape is unknown in most cases. In some Gymnolaemata, polypides which develop an ovary possess a flask-shaped “intertentacular organ,” situated between two of the tentacles, and affording a direct passage into the introvert for the eggs or even the spermatozoa developed in the same zooecium. In other cases the reproductive cells perhaps pass out by the atrophy of the polypide, whereby the body-cavity may become continuous with the exterior. The statoblasts of the Phylactolaemata originate on the funiculus, and are said to be derived partly from an ectodermic core possessed by this organ and partly from its external mesoderm (Braem), the former giving rise to the chitinous envelope and to a nucleated layer (fig. 7, ect), which later invaginates to form the inner vesicle of the polypide-bud. The mesodermic portion becomes charged with a yolk-like material (y), and, on the germination of the statoblast, gives rise to the outer layer (mes) of the bud. The production of a polypide by the statoblast thus differs in no essential respect from the formation of a polypide in an ordinary zooecium. The statoblasts require a period of rest before germination, and Braem has shown that their property of floating at the surface may be beneficial to them by exposing them to the action of frost, which in some cases improves the germinating power. The occurrence of Phylactolaemata in the tropics would show, however, without further evidence, that frost is not a factor essential for germination.

The withdrawal of the extended polypide is effected by the contraction of the retractor muscles (fig. 6, mr), and must result in an increase in the volume of the contents of the body-cavity. The alternate increase and diminution of volume is easily understood in forms with flexible zooecia. Thus in the Phylactolaemata the contraction of the muscular body-wall exerts a pressure on the fluid of the body-cavity and is the cause of the protrusion of the polypide. In the Gymnolaemata protrusion is effected by the contraction of the parietal muscles, which pass freely across the body-cavity from one part of the body-wall to another. In the branching Ctenostomes the entire body-wall is flexible, so that the contraction of a parietal muscle acts equally on the two points with which it is connected. In encrusting Ctenostomes and in the Membranipora-like Cheilostomes (figs. 8 A, 9 A) the free surface or frontal wall is the only one in which any considerable amount of movement can take place. The parietal muscles (p.m.), which pass from the vertical walls to the frontal wall, thus act by depressing the latter and so exerting a pressure on the fluid of the body-cavity. In Cheilostoma with a rigid frontal wall Jullien showed that protrusion and retraction were rendered possible by the existence of a “compensation-sac,” in communication with the external water.

In its most fully developed condition (fig. 9, C) the compensation-sac (c.s.) is a large cavity which lies beneath the calcified frontal wall and opens to the exterior at the proximal border of the operculum (fig. 10). It is joined to the rigid body-wall by numerous muscle-fibres, the contraction of which must exert a pressure on the fluid of the body-cavity, thereby protruding the polypide. The exchange of fluid in the sac may well have a respiratory significance, in addition to its object of facilitating the movements of the tentacles.

The evolution of the arrangements for protruding the polypide seems to have proceeded along several distinct lines: (i.) In certain

species of Membranipara the “frontal membrane,” or membranous free-wall, is protected by a series of calcareous spines, which start from its periphery and arch inwards. In Cribrilina similar spines are developed in the young zooecium, but they soon unite with one another laterally, leaving rows of pores along the sutural lines (fig. 10). The operculum retains its continuity with the frontal membrane (fig. 9, B) into which the parietal muscles are still inserted. As indications that the conditions described in Membranipora and Cribrilina are of special significance may be noted the fact that the ancestrula of many genera which have well-developed compensation-sacs in the rest of their zooecia is a Membranipora-like individual with a series of marginal calcareous spines, and the further fact that a considerable proportion of the Cretaceous Cheilostomes belong either to the Mernbraniporidae or to the Cribrilinidae. (ii.) In Scrupocellaria, Menipea and Caberea a single, greatly dilated marginal spine, the “scutum” or “fornix,” may protect the frontal membrane. (iii.) In Umbonula the frontal membrane and parietal muscles of the young zooecium are like those of Membranipora, but they become covered by the growth, from the proximal and lateral sides, of a calcareous lamina covered externally by a soft membrane. The arrangement is perhaps derivable from a Cribrilina-like condition in which the outer layer of the spines has become membranous while the spines themselves are laterally united from the first. (iv.) In the Microporidae and Steganoporellidae the body-cavity becomes partially subdivided by a calcareous lamina (“cryptocyst,” Jullien) which grows from the proximal and lateral sides in a plane parallel to the frontal membrane and not far below it. The parietal muscles are usually reduced to a single pair, which may pass through foramina (“opesiules”) in the cryptocyst to reach their insertion. There is no compensation-sac in these families. (v.) Many of the Lepralioid forms offer special difficulties, but the calcareous layer of the frontal surface is probably a cryptocyst (as in fig. 9, C), the compensation-sac being developed round its distal border. The “epitheca” noticed above is in this case the persistent frontal membrane. (vi.) In Microporella the opening of the compensation-sac has become separated from the operculum by calcareous matter, and is known as the “medianipore.” Jullien believed that this pore opens into the tentacle-sheath, but it appears probable that it really communicates with the compensation-sac and not with the tentacle-sheath. The mechanism of protrusion in the Cyclostomata is a subject which requires further examination.

The most singular of the external appendages found in the Polyzoa are the avicularia and vibracula of the Cheilostomata. The avicularium is so called from its resemblance, in its most highly differentiated condition, to the head of a bird. In Bugula, for instance, a calcareous avicularium of this type is attached by a narrow neck to each zooecium. The avicularium can move as a whole by means of special muscles, and its chitinous lower jaw