Fig. 75. An advanced larva of Epibulia aurantiaca with one large nectocalyx. (After Metschnikoff.)
so. somatocyst; nc. second imperfectly developed nectocalyx; hph. hydrophyllium; po. polypite; t. tentacle.
The former of these elongates itself in succeeding stages into a process of both epiblast and hypoblast. The central part of the nectocalyx on the other hand appears to originate from a thickening of the epiblast in which the cavity of the bell becomes subsequently hollowed out. Between this part and the external epiblast which gives origin to the outermost layer of the nectocalyx a layer of hypoblast is interposed. When the nectocalyx has become to a certain extent established a cavity—the commencement of the primitive gastrovascular cavity of the adult—appears in the general hypoblast between the epithelial and nutritive layers in the immediate neighbourhood of its attachment. This cavity becomes prolonged into the nectocalyx to form the four gastrovascular canals; while the hypoblast at the upper end of the nectocalyx forms the somatocyst ([fig. 74] C, so). The primitive enteric cavity once formed rapidly extends, especially in an oral direction ([fig. 74] C), and forms a widish cavity in the oral part of the embryo. At the pole of this part ([fig. 74], po) is eventually formed the opening of the mouth, and the contained cavity becomes in a special sense the gastric cavity. This region of the embryo may be spoken of as the polypite. The nectocalyx grows with great rapidity and soon forms by far the most prominent part of the larva ([fig. 75]). The true gastric region or polypite ([fig. 75], po) continues also to grow, and a mouth becomes formed at its extremity. The aboral end of the original body of the embryo gradually atrophies.
Fig. 76. Two stages in the development of Stephanomia pictum.
(After Metschnikoff.)
A. Stage after the delamination. ep. epiblastic invagination to form pneumatocyst.
B. Later stage after the formation of the gastric cavity in the solid hypoblast, po. polypite; t. tentacle; pp. pneumatophore; ep. epiblastic invagination to form pneumatocyst; hy. hypoblast surrounding pneumatocyst.
At the junction of the nectocalyx and polypite the cœnosarc becomes formed, and rudiments of a second nectocalyx (nc) and second polypite early become visible; while a hydrophyllium is formed as a bud which covers over the first polypite and tentacle (hph). With the development of the hydrophyllium the first segment, if the term may so be used, is complete. The second segment of which a rudiment is already present as a second polypite is intercalated between the first segment and the nectocalyces.