Thin transverse section of Halodactylus diaphanus.

Fig. 240. Thin transverse section of Halodactylus diaphanus. The centre occupied by cellular tissue and water; the circumference formed by cells in close approximation; the brown bodies scattered through the substance. a a, Position of the gemmules enclosed in their sac; 6, one of the gemmules escaped during the section of the central tissue.

* Dr. A. Farre, Phil. Trans. 1837,-p. 140.

(1243). We have hitherto only spoken of those Polyzoa whose habitat is the sea; besides the marine genera, there are, however, many individuals belonging to this class that abound in fresh water. The polyparies of the Fluviatile Polyzoa are met with in ponds and streams, adherent to any foreign bodies which may be casually submerged*. Thus, they are found attached to stones at the bottom of the water; upon the shells of Anodon, Unio, and other freshwater mollusca; upon leaves, more especially those of the Water-lily (Nymphcea) and of the Bistort (Polygonum amphibium); upon floating wood; upon the stems of Arundo phragmites and of various other plants. Some genera (Alcyonella and Fredericella) frequently agglomerate into masses of considerable size, such as might be mistaken for spongillee. The Paludd-cellce often form an inextricable interlacement of filaments, spread out over shells and stones. Cristatella and Lojphopus are generally met with upon the stem of some aquatic plant, such as the Brook-lime ( Veronica beccabunga), resembling, when examined by the naked eye, a layer of fluid albumen, which might easily be mistaken for the eggs of Lim-nceus stagnalis.

In order to examine these animals in a living state, it is necessary to leave the leaf to which they are attached for some time undisturbed in a glass of clear water, when they will soon be seen spreading forth their beautiful tentacula as they protrude from their delicate cells. By frequently changing the water, more especially if it is rich in Navicular and Bacillariae, they may be kept alive for months, affording objects of continual interest for the microscope.

(1244). In the freshwater Polyzoa the structure of the external envelope is similar to that of the marine species, except that in no instance are the fluviatile genera known to possess a calcareous polypary.

(1245). In Cristatella mucedo1 (fig. 241, 3) the polypary or external envelope (d) is membranous and slightly cordiform, its surface is tuberculated, and it is incapable of contraction. In this outer covering several individuals are contained; but although produced from one another, they are only aggregated, being lodged in distinct tubular cells. The body of each animal appears to consist of a digestive canal constricted once or twice in its course, and terminated by an anal orifice. When these creatures are extended, the upper part of the body protrudes from the cell, the tentacular apparatus being supported on a kind of neck, whereon the mouth (a) is easily seen, and near it the anus.

* "Recherches sur les Bryozoaires fluviatiles de Belgique," par P. J. Van Beneden (Nouv. Mem. de l'Acad. de Bruxelles, 1847).

1 M. Turpin, "Etude mieroscopique de la Cristatella mucedo, espece de polype d'eau douce" (Ann. des Sci. Nat. for 1837.) Also, another memoir upon the same subject by M. P. Gervais (ibid.).

(1246). On each side of the mouth the body divides into two arms, which, when spread out, resemble a horseshoe, being flattened and blunt; and upon the arms are arranged about a hundred slender, transparent, retractile tentacles, disposed on each side and upon the summit like the barbs of a feather, and all covered with an infinite number of cilia, whose action produces currents directed towards the mouth, hurrying in that direction organized particles contained in the water.

(1247). The three individuals that thus inhabit the same general covering are produced at two distinct generations - the two lateral being the offspring of the central one, derived from it by a process of gemmation; but, when complete, they are evidently quite separate from and independent of their parent.

(1248). The number of the tentacular appendages varies very considerably in different genera: in Paludieella and Fredericella, which have the fewest, there are about twenty, while in Alcyonella, Plumatella, and Cristatella (fig. 241) there are as many as sixty, or even more. In Paludieella the arrangement of the tentacula is infundibular; but in Lophojpus, Alcyonella, Plumatella, and Cristatella (fig. 241) they assume the shape of a horse-shoe. In Fredericella (fig. 242) they are united together for one-half of their length by means of a delicate membrane.

(1249). The digestive apparatus in all these different genera consists of an oesophagus, of a stomach, which forms a cul de sac, and intestinal tube. The intestine is always straight, and without convolutions. Its cavity is separated from that of the stomach by a pyloric valve that completely closes the aperture; whilst the oesophagus is in like manner provided with a fold, situated sometimes near its middle, sometimes at its lower part, that performs the office of a cardiac valve.

Cristatella mucedo. 1. Egg, natural size. 2. Egg, magnified.

Fig. 241. Cristatella mucedo. 1. Egg, natural size. 2. Egg, magnified. 3. Animal after its escape from the egg: a, the mouth; b, openings of cell; c, the stomach; d, shell; e,f, ciliated tentacula.

(1250). The aliments,before admission into the stomach, accumulate in a cavity formed at the commencement of the digestive tube (fig. 242,f), which in most genera is defended by a largely developed lip (d) that opens and shuts like a valve; this lip is densely covered with cilia, the action of which is very energetic.