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.
In both the groups (Tubularia and Hydra) which are exceptional in not having a ciliated planula stage, its absence may be put down to an abbreviation of the development, and in fact a two-layered quiescent stage, through which the embryo passes, may be regarded as representing the planula stage.