Fig. 81. Larva of Astroides calycularis shortly after it has become attached. (After Lacaze Duthiers.)
The figure shews the development of the Y-shaped septa in the intervals between the mesenteries. The position of the latter is indicated by the faint shading. The theca has become developed externally.
After the formation of the theca the septa become divided into two cycles by the predominant growth of six of them. On the coalescence of the septa with the theca the space between the two limbs of the Y becomes filled up with calcareous tissue. The law of the formation of the third cycle of septa (12‑24) has not been worked out, so that it is not possible to state whether it follows the peculiar principles regulating the growth of the tentacles.
The whole of the skeletal parts occupy a position between the epiblast and hypoblast, and are exactly homologous in this respect with the skeleton of the Alcyonidæ. By Lacaze Duthiers they are however believed to originate in the hypoblast, but from the observations of Kowalevsky there be little doubt that they arise in the connective tissue between the two embryonic layers which is probably epiblastic in origin.
A peculiar larva, probably belonging to the Actinozoa, has been described by Semper[83]. It has an elongated form and is provided with a longitudinal ridge of cilia. There is a mouth at one end of the body and an anus at the opposite extremity. The mouth leads into an œsophagus, which opens freely into a stomach with six mesenteries. In the skin are numerous thread-cells. A mesotrochal worm-like larva, also provided with thread-cells, and found at the same time, was conjectured by Semper to be a younger form of this larva.
Ctenophora. The ovum of the Ctenophora is formed of an outer granular protoplasmic layer and an inner spongy mass with fatty spherules. It is enveloped in a delicate vesicle, the diameter of which is very much greater than that of the contained ovum. This vesicle appears to be filled with sea-water, in which the ovum floats.
Fertilized ova may usually be easily obtained by keeping the captured adults in water from 12‑24 hours. The two main authorities on the development of these forms (Kowalevsky, No. [147] and [178] and Agassiz, No. [172]) are unfortunately at variance on one or two of the most fundamental points. It seems however that the embryonic layers are formed by a kind of epibolic gastrula; while the true gastric cavity, as distinct from the gastrovascular, is formed by an invagination, and deserves therefore to be regarded as a form of stomodæum.
Fig. 82. Five stages in the development of Idyia roseola.
(After Agassiz.)