(1251). The anus (j) is always situated at the base of the tentacular zone.

(1252). The food of the fluviatile Polyzoa consists of Infusorial animalcules and the microscopic Desmidieae which abound in the waters they frequent, and whose remains are distinguishable both in their stomachs and in the contents of the intestine. They are likewise easily made to swallow carmine, sepia, and other colouring substances.

(1253). There seems to be no doubt relative to the nature of the circulation in these animals. The place of blood seems to be altogether supplied by the chylaqueous fluid. This fluid is not contained in vessels, unless the cavities of the tubular tentacula be considered as such, but moves freely in all directions around the parietes of the digestive canal. There is consequently neither heart nor any vascular system, - the chylaqueous fluid, which thus represents the blood, being kept in continual movement in the periintes-tinal cavity by the action of the cilia that cover the exterior of the intestinal apparatus. It is therefore ciliary action that determines the course of the aliment in the interior of the alimentary apparatus, and of the fluid external to its walls - the cilia thus answering the purpose of a heart as well as of the muscular coat of the intestines.

(1254). All the viscera of the body being thus bathed by the chylaqueous fluid that surrounds the intestinal canal, they receive directly, through the intermedium of that fluid, both the materials for nourishment and the means of respiration.

(1255). Reproduction among the freshwater Polyzoa is accomplished in two ways - by gemmation and by true ova. The first of these modes resembles exactly what has been described as existing among the marine genera; but as regards the process of oviparous reproduction, there are some remarkable points of difference that require notice.

Fredericella sultana: d, mouth; f, pharynx; j, anal orifice.

Fig. 242. Fredericella sultana: d, mouth; f, pharynx; j, anal orifice.

(1256). It is now generally understood that, wherever oviparous reproduction occurs, there is a formation of spermatozoa; and modern observations have proved the existence of these distinguishing products of the male sex in most genera of the ciliobrachiate polyps. Frequently both the sexes are conjoined, so that there is a complete hermaphroditism; but in some cases the sexes are separate, and the number of female individuals is greater than that of the males. Seeing that among these compound animals the blood, or its representative fluid, is common to an entire group, and that the ova as well as the spermatozoa are diffused through this liquid before they are evacuated, a single male individual may, strictly speaking, suffice for the fecundation of the eggs of a whole colony.

(1257). But with regard to the ova themselves, a remarkable difference is observable between those of the fluviatile and of the marine Polyzoa. Among the Alcyonellce and other genera there exist two sorts of eggs - the one covered with vibratile cilia, capable of swimming freely about exactly like Infusorial animalcules, and the other enclosed in a hard shell, having somewhat the appearance of the seeds of some plants. The first sort, without a shell, is also met with among marine species; but the second seems peculiar to the freshwater Polyzoa. In Crista-tella mucedo, for example, the ova are of this latter description, being enclosed in a dense horny shell, the exterior of which is covered over with sharp hooklets, giving them an appearance strikingly like some of the Desmidieae (fig. 241, 2.) This shell is probably intended to preserve the ova during the winter season from being destroyed by the freezing of the ponds in which they occur, while the marine polyps, being subjected to no such changes of temperature, can dispense with such a covering. It is on this account apparently that these ova are met with sometimes naked, and sometimes provided with a shell; and in the same way, in the genus Paludicella (in which ova have not been detected), the gemmae become invested on the approach of winter with a horny covering.

(1258). As there are thus two modes of reproduction, so are there two kinds of embryogenic development observable among the fluviatile Polyzoa; that is to say, the polyp which is produced from an egg is formed in a different manner from that which is produced by the process of gemmation. In the mature ovum, both the germinal spot and the germinal vesicle are distinctly perceptible; but in the nascent gemma the existence of neither of these elements is to be detected. According to the gemmiparous mode of propagation, the young individual is formed by direct extension from the tissues of the parent. In the formation of the embryo from an egg, there is, from the first, a complete isolation of the newly-formed progeny: a vesicle or cell is formed, which, previous to its conversion into a new individual, requires the cooperation of another cell, or, in other words, the ovum remains unproductive unless brought in contact with the male fluid containing spermatozoa, whereas in gemmiparous reproduction such a concurrence is by no means necessary; neither germinal vesicle nor any male apparatus is required.