Among the Ascidians this general law of morphological composition is once more displayed. Each of these creatures subsists on the nutritive particles contained in the water which it draws in through one orifice and sends out through another; and it may thus subsist either alone, or in connexion with others that are in some cases loosely aggregated and in other cases closely aggregated. Fig. 156, _Phallusia mentula_, is one of the solitary forms. A type in which the individuals are united by a stolon that gives origin to them by successive buds, is shown in _Perophora_, Fig. 157. Among the _Botryllidæ_, of which one kind is drawn on a small scale in Fig. 159, and a portion of the same on a larger scale in Fig. 158, there is a combination of the individuals into annular clusters, which are themselves imbedded in a common gelatinous matrix. And in this group there are integrations even a stage higher, in which several such clusters of clusters grow from a single base. Here the compounding and recompounding appears to be carried further than anywhere else in the animal kingdom. Thus far, however, among these aggregates of the third order, we see what we before saw among the simpler aggregates of the second order--we see that the component individualities are but to a very small extent subordinated to the individuality made up of them. In nearly all the forms indicated, the mutual dependence of the united animals is so slight, that they are more fitly comparable to societies, of which the members co-operate in securing certain common benefits. There is scarcely any specialization of functions among them. Only in the last type described do we see a number of individuals so completely combined as to simulate a single individual. And even here, though there appears to be an intimate community of nutrition, there is no physiological integration beyond that implied in several mouths and stomachs having a common vent.[20]