A number of minute parasites infesting various Nemertines, Turbellarians, and Ophiuroids have recently been studied by Giard and Metschnikoff, the former of whom has placed them in a special group which he calls the Orthonectidæ. They were first discovered by W. C. McIntosh.
In the adult state they are[66] (Metschnikoff) somewhat pear-shaped bodies formed of a kind of plasmodium of cells with irregular lobate processes. In the interior of this body are eggs in all stages of development. In the type observed by Metschnikoff (Intoshia gigas) the ova undergo a regular segmentation, resulting in the formation of a blastosphere in which an inner layer is subsequently formed by delamination. A smaller and a larger kind of embryo are formed; but all the embryos in each female belong to one type. The larger become females and the smaller males.
The female embryos are ovoid. The outer layer of cells or epiblast becomes ciliated, and divided into nine segments, of which the second is marked off from the remainder by the absence of cilia, and by being provided with refractive corpuscles. The inner layer which surrounds a central cavity, and might be supposed to be the hypoblast, becomes according to Metschnikoff converted into ova.
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.
(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.