The embryo, in the cases in which it is still contained within the sporosack, now begins to exhibit slight changes of form, and one extremity of it begins to elongate. It soon becomes free, and rapidly assumes an elongated cylindrical form, while a coating of cilia, by means of which it moves sluggishly about, appears on its outer surface. A central cavity appears in the interior, and the inner cells form themselves into a definite hypoblast. The larva has now become a planula, and consists of a closed sack with double walls. It continues for some few days to move about, but eventually drops its cilia, and becomes dilated at one extremity, by which it then becomes attached. The base of attachment becomes gradually enlarged so as to form a disc, which spreads out and is frequently divided by fissures into radiating lobes. The free extremity becomes enlarged to form the eventual calyx. Over the whole exterior a delicate pellicle--the future perisarc--now becomes secreted. Round the edge of the anterior enlargement a row of tentacles makes its appearance. These, in the embryos of the Tubularian genera, lie some little way behind the apex of the body. After a certain time the perisarc, which has hitherto been continuous, becomes ruptured in the region of the calyx, and the tentacles become quite free. At about the same period a mouth is formed at the oral apex. [FIG. 68. THREE LARVA STAGES OF EUCOPE POLYSTYLA. (After Kowalevsky.) A. Blastosphere stage with hypoblast spheres becoming budded off into the central cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric cavity. _ep._ epiblast; _hy._ hypoblast; _al._ gastric cavity.] The development of Eucope polystyla (fig. 68), one of the Campanularidæ, deviates according to Kowalevsky (No. 147) in somewhat important points from the usual type. The whole development takes place after the deposition of the ovum. The segmentation results in the formation of a single-walled blastosphere with a large central cavity (fig. 68 A). This cavity, somewhat as in Ascetta, becomes filled up with a not clearly (?) cellular material derived from the walls of the blastosphere, which must be regarded as the hypoblast (fig. 68 B). The larva elongates and becomes ciliated, and the epiblast at its two extremities becomes thickened, and is stated by Kowalevsky also to become divided into two layers. The alimentary cavity appears as a slit in the middle of the hypoblast (fig. 68 C). The cilia after a time disappear, and the larva then becomes fixed by one extremity. It flattens itself out into a disc-like form, becomes divided into four lobes, and covered by a cuticle (perisarc). From the disc the stalk grows out which dilates at its free extremity into the calyx. [FIG. 69. LONGITUDINAL SECTION THROUGH A LARVA OF TUBULARIA MESEMBRYANTHEMUM WHILE STILL IN THE GONOPHORE. The lower end is the oral one. _ep._ epiblast; _hy._ hypoblast of tentacle; _en._ enteric cavity.]