For general account of Orthonectidæ, vide Spengel. Biolog. Centralblatt, No. 6.]
[66] This at any rate holds true for the type investigated by Metschnikoff. The full history of other forms is not yet known.
CHAPTER V.
PORIFERA.
Although within the last few years greater advances have probably been made in our knowledge of the development of the Porifera than of any other group, yet there is much that is still very obscure, and it is not possible to make general statements applying to the whole group.
Calcispongiæ. The form which has so far been most completely worked out is Sycandra raphanus, one of the Calcispongiæ (Metschnikoff, Nos. [132] and [134], F. E. Schulze, Nos. [139] and [142]), and I shall commence my account with the life history of this species.
The ovum in Sycandra as in other Spongida has the form of a naked amœboid nucleated mass of protoplasm. From the analogy of the other members of the group, there is no doubt that it is fertilized by a male spermatic element, though this has not as yet been shewn to be the case—and the changes which accompany fertilization are quite unknown.
