The male embryos are more elongated than the female, from which they further differ in only having six segments. The cells of the inner layer eventually divide up into spermatozoa. The larvæ probably become free, and while in the free state impregnation would appear to be effected. When the female larvæ become parasitic they undergo a metamorphosis, the stages of which have not been observed; but in the course of which the epiblast cells probably unite into a plasmodium. The observations of Giard are in several points irreconcilable with those of Metschnikoff, but from the statements of the latter it appears possible that Giard has made two genera from the males and females of one species; and that Giard's account of an unequal segmentation followed by an epibolic gastrula, in one of his species, has arisen from two segmenting ova temporarily fusing together. Giard has given a description of internal gemmiparous reproduction, upon the accuracy of which doubts have been thrown by Metschnikoff. The affinities of the Orthonectidæ are as obscure as those of the Dicyemidæ; though there can be but little doubt that their organization has been much simplified in correlation with their parasitic habits. The origin of the genital products in the axial tissue is a feature they have in common with the Dicyemidæ. BIBLIOGRAPHY. (120) Alf. Giard. "Les Orthonectida classe nouv. d. Phylum des Vers." _Journal de l'Anat. et de la Physiol._, Vol. XV. 1879. (121) El. Metschnikoff. "Zur Naturgeschichte d. Orthonectidæ." _Zoologischer Anzeiger_, No. 40-43, 1879. [Ch. Julin. "Rech. sur l'organization et le devel. d'Orthonectides." _Arch. Biol._ Vol. III. 1882. E. Metschnikoff. "Untersuchungen üb. Orthonectidæ." _Zeit. f. Wiss. Zoologie_, Vol. XXXV. 1881. For general account of Orthonectidæ, _vide_ Spengel. _Biolog. Centralblatt_, No. 6.] 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 amoeboid 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. [FIG. 63. SUCCESSIVE STAGES IN THE SEGMENTATION OF SYCANDRA RAPHANUS. (Copied from F. E. Schulze.)