[Illustration: FIG. 46.—A, Ascidiozooid from a Botryllid colony; B, ascidiozooid from a Distomid colony; C, ascidiozooid from a Polyclinid colony. _a_, Anus; _at_, atrial aperture; _at.l_, atrial languet; _br_, branchial aperture; _cl_, cloaca; _d.l_, dorsal languet; _ec_, ectoderm; _end_, endostyle; _ep.c_, epicardiac tube; _gl_, intestinal gland; _h_, heart; _i_, intestine; _n.g_, nerve-ganglion; _oes_, oesophagus; _ov_, ovary; _p.c_, pericardium; _r_, rectum; _sg_, stigmata of branchial sac; _sp_, spermatic sacs; _sph_, sphincter; _st_, stomach; _t_, tentacle; _t.k_, terminal ampullae of vessels in test; _v_, colonial vessels; _v.app_, "vascular appendage" (stolon).] The marked differences in the appearance of the colonies of Compound Ascidians is largely due to the methods of budding; and even in those of the stolonial type, where the budding is practically the same in essential nature, the results may be very different in superficial appearance, according as the buds are {83}formed on a short stolon close to the parent body, or from the extremity of the post-abdomen (as in the Polyclinidae), or from a long epicardiac tube (as in _Colella_, Fig. 47), which may extend for some inches from the ascidiozooid. The post-abdomen of the Polyclinidae may be regarded as a stolon invaded by the gonads and the heart (see Fig. 46, C), and traversed by the epicardium in the form of a flattened tube dividing a dorsal blood-sinus containing the gonads from a ventral sinus which has merely the one extremity of the tapering pericardium. The whole of this post-abdomen segments to form the buds, the heart at the extremity being absorbed, and a new one formed from the anterior end of the pericardium. The epicardium, which supplies the endodermal element to each bud, was first described by E. van Beneden and Julin in the envelopment of _Clavelina_,[99] as a structure concerned in the formation of the pericardium and heart—hence its unfortunate name. It grows backwards in the larva, from the posterior wall of the branchial sac, close to the endostyle, as a tube which usually divides into two lateral branches to be united again eventually so as to form the single tubular flattened partition of the stolon in Polyclinidae, Distomatidae, Clavelinidae, etc. In some Compound Ascidians the epicardium is, from its origin, two distinct lateral tubes, which grow back from the inner vesicle of the embryo (later the branchial sac). These unite in the post-abdomen to form the flattened tube, which in its turn forms the inner vesicle of the future buds, and so the endodermal element is handed on from generation to generation. In addition to the epicardium, the stolon contains also a prolongation of the ovary of the parent, or at least a string of migrating germ cells, so that the reproductive elements are also handed on.