In such a colony of connected individuals, the exact limits of the separate "persons" are not always clearly marked out. Hence it is necessary to distinguish between, first, the "zooids," indicated in the case of the polyps by the hydranths, each with mouth and tentacles; and, secondly, the "coenosarc," or common flesh, which cannot be assigned more to one individual than another, but consists of a more or less complicated network of tubes, corresponding to the hydrocaulus and hydrorhiza of the primitive independent polyp-individual. The coenosarc constitutes a system by which the digestive cavity of any one polyp is put into communication with that of any other individual either of the trophosome or gonosome. In this manner the food absorbed by one individual contributes to the welfare of the whole colony, and the coenosarc has the function of circulating and distributing nutriment through the colony. The hydroid colony shows many variations in form and architecture which depend simply upon differences in the methods in which polyps are budded. [Illustration: After Hincks, Forbes, and Browne. A and B modified from Hincks; C modified from Forbes's _Brit. Naked-eyed Medusae_. FIG. 11.--_Lar sabellarum_ and two stages of its Medusa, _Willia stellata_. A, colony of _Lar_; B and C, young and adult medusae.] In the first place, buds may be produced only from the hydrorhiza, which grows out and branches to form a basal _stolon_, typically net-like, spreading over the substratum to which the founder-polyp attached itself. From the stolon the daughter-polyps grow up vertically. The result is a spreading or creeping colony, with the coenosarc in the form of a root-like horizontal network (fig. 5, B; 11, A). Such a colony may undergo two principal modifications. The meshes of the basal network may become very small or virtually obliterated, so that the coenosarc becomes a crust of tubes tending to fuse together, and covered over by a common perisarc. Encrusting colonies of this kind are seen in _Clava squamata_ (fig. 5, A) and _Hydractinia_ (figs. 9, 10), the latter having the perisarc calcified. A further very important modification is seen when the tubes of the basal perisarc do not remain spread out in one plane, but grow in all planes forming a felt-work; the result is a massive colony, such as is seen in the so-called Hydrocorallines (fig. 60), where the interspaces between the coenosarcal tubes are filled up with calcareous matter, or _coenosteum_, replacing the chitinous perisarc. The result is a stony, solid mass, which contributes to the building up of coral reefs. In massive colonies of this kind no sharp distinction can be drawn between hydrorhiza and hydrocaulus in the coenosarc; it is practically all hydrorhiza. Massive colonies may assume various forms and are often branching or tree-like. A further peculiarity of this type of