Fig. 33, M, has been drawn from a figure published by Ellis in his _Natural History of the Corallines_ published in 1755[307]. The thallus has the form of a repeatedly forked body, of which the branches are divided into cylindrical joints thickly encrusted with carbonate of lime, but constricted and uncalcified at the limits of each segment. A tuft of hairs is given off from the terminal segment of each branch. The axis of each branch of the thallus is occupied by a cylindrical and unseptate cell which gives off crowded whorls of lateral branches. In the lower part of fig. 33, M, the calcareous investment has been removed, and the branches are seen as fine hair-like appendages of the central cell. The branches given off from the constricted portions of the axis are unbranched simple appendages, but the others terminate in bladder-like swellings, each of which bears an apical sporangium. The sporangia are surrounded and enclosed by the swollen tips of four to six branches which spring from the summit of the sporangial branch. Fig. 33, A, represents part of a transverse section through the calcareous outer portion of a branch of _Cymopolia_; the darker portions or cavities in the calcareous matrix were originally occupied by the lateral branches and sporangia[308]. In Fig. 33, B, the sporangial branch with the terminal sporangium and three of the investing branches are more clearly shown, the surrounding calcareous investment and the thallus having been removed by the action of an acid. In a transverse section of a branch from which the organic matter had been removed, and only the calcareous matrix left, one would see a central circular cavity surrounded by a thick calcareous wall perforated by radially disposed canals and containing globular cavities; the canals and cavities being occupied in the living plant by branches and sporangia respectively. The two circular cavities shown in the figure mark the position of the sporangia which are borne on branches with somewhat swollen tips. From the summit the left-hand sporangial branch shown in fig. 33, A, three of the secondary branches are represented by channels in the calcareous matrix; the two black dots on the face of the sporangiophore being the scars of the remaining two secondary branches. By the lateral contact of the swollen ends of the ultimate branches enclosing the sporangia the whole surface of the thallus, when examined with a lens, presents a pitted appearance. Each pit or circular depression (fig. 33, N) marks the position of the swollen tip of a branch.