136 ORTHONECTIM:. 



ORTHONECTIM;. 



A number of minute parasites infesting various Nemertines, Turbella- 

 rians, 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 

 Orthonectidae. They were first discovered by W. C. Mclntosh. 



In the adult state they are 1 (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 larva} probably become free, and while in the free state impregna- 

 tion would appear to be effected. When the female larvae 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 plasmo- 

 dium. 



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 Orthonectida: are as obscure 

 as those of the Dicyemida? ; 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 Dicyemidae. 



1 This at any rate holds true for the type investigated by Metschnikoff. The 

 full history of other forms is not yet known. 



