This is probably a somewhat artificial assemblage formed of those two or three groups of Ascidians which produce colonies, in which the ascidiozooids are so intimately united that they possess a common test or investing mass. This is the only character which distinguishes them from the Clavelinidae, but the property of reproducing by gemmation separates them from the rest of the Ascidiae Simplices. In some cases the atrial apertures of several neighbouring ascidiozooids join to open to the exterior by a common cloacal aperture (Fig. 45, _c.c_). Such {81}groups of the ascidiozooids of a colony are known as "systems" or coenobia (see Fig. 44, D; also Fig. 53, p. 89). The Ascidiae Compositae may be divided into seven families, which seem to fall into two well-marked sets:—(1) MEROSOMATA, in which the heart and alimentary and reproductive viscera are placed behind the branchial sac, so as to constitute a more or less extended body divided into at least two regions (Fig. 46, B), and sometimes three (Fig. 46, C)—thorax, abdomen, and post-abdomen; and (2) HOLOSOMATA, in which the body of the ascidiozooid is short, compact, and not divided into regions (Fig. 46, A). The latter group comprises the two families Botryllidae and Polystyelidae, which agree both in points of structure and in having the same type of budding, and are probably derived from ancestral Cynthiidae amongst Simple Ascidians; while the Merosomata seem more nearly related to the Clavelinidae. [Illustration: FIG. 45.—Vertical section through a small part of a compound Ascidian colony. _Asc_. 1 and _Asc_. 2, Parts of two ascidiozooids whose cloacas (_cl_) open into the common cloacal cavity (_c.c_) of the colony; _at.l_, atrial lobes; _t_, _t_, _t_, common test of the colony. The structure of the posterior parts of the ascidiozooids would depend upon the family (see Fig. 46). The arrows show the direction of the water currents.] GEMMATION takes place in the Compound Ascidians in a variety of ways, being sometimes very different in its details in closely allied forms. There are, however, two main types of budding, to one or other of which most of the described methods may be referred. These are:— 1. The STOLONIAL, or "epicardiac" type—seen in the Merosomata, typically in Distomatidae and Polyclinidae, and comparable with the gemmation in Clavelinidae, Pyrosomatidae, and Thaliacea outside this group. {82}2. The PARIETAL, or "peribranchial" type—seen in the Holosomata, typically in the Botryllidae. The remarkable process of gemmation seen in the families Didemnidae and Diplosomatidae, where the bud arises from at least two rudiments, the one stolonial or epicardiac in origin, and the other formed by one or more oesophageal or intestinal outgrowths, has been called "entero-epicardiac," but it may probably be regarded as a modification of the stolonial type.