It is believed that the polypes of the Polyzoa are male and female, and that the ciliated locomotive larvæ which appear in spring are developed from eggs. The fresh-water Polyzoa are as worthy of microscopic examination as the marine. SECTION X. TUNICATA, OR ASCIDIANS. THE form of the Tunicata is irregular. They have two orifices—one at the top, for the entrance of a current of water, and another at one side for its egress. They have two tunics only adhering to one another at the edges of these orifices, which are furnished with a circle of cilia. The irregular or scattered condition of the nerve-centres, as well as the alternation in the circulation, are eminently characteristic of the whole class. They consist of three distinct groups: the compound or social gelatinous Ascidians; the solitary Tunicata, with leathery coats; and the Salpæ, which are gelatinous. The two first, though mobile when young, become fixed when they arrive at maturity; the third floats free on the surface of the ocean. [Illustration: Fig. 160. Magnified group of Perophora.] The fixed gelatinous Ascidians resemble the Polyzoa in structure and tendency to gemmation; nevertheless, they differ in their circulating and respiratory systems. The Perophora Listeri is an example which is found on the south coast of England and Ireland (fig. 160.) It consists of minute globes of clear jelly, not larger than a pin’s head, spotted with orange and brown, and attached by a foot-stalk to a silvery stem like a thread which stretches over the surface of stones, or twines round the stalks of sea-weeds; and as the stem increases in length, buds spring from it, which in time come to maturity, so that the silvery thread connects a large community; but, though thus connected, every member has its own individuality. Fig. 161 represents one of these transparent individuals very highly magnified. [Illustration: Fig. 161. Highly magnified Perophora.] The respiratory sac occupies the upper part of the body. It is perforated by four rows of narrow slits, edged with cilia, whose vibrations are distinctly seen through the transparent tunic of the little animal. A portion of the water which is drawn by the cilia into the upper orifice or mouth, passes into the respiratory sac, escapes through the narrow slits into the space between the sac and the tunic, and from thence into the stomach, where any particles of food it may bring are digested, and the refuse is carried by the current through the intestinal canal, and ejected at the lateral orifice.