THE ORTHONECT1DA 



155 



derived from the " intermediate cells," which have been sometimes noted 

 in these positions. It has, however, been suggested that the " fibrillae " 

 are merely the expressions of a folding of the testis-membrane. 



The female is not only larger than the male, but differs so greatly 

 from it that Giard referred it to a different genus, and described it under 

 the name Intoshia gigas. Further, Julin discovered that there are two 

 kinds of females one flat, the other cylindrical. The cylindrical female 

 presents eight rings (or, according to Metschnikoff, nine), (Fig. VIII.). Of 

 these the second, as in the male, is not ciliated. The number of rows 

 of cells differs in the various rings. 



The flattened female (Fig. IX.) is broader than the cylindrical form, and 

 presents no ectodermal rings. The ectoderm is formed of very flat cells, 

 and is ciliated over its entire surface. There is at one point a sub- 

 epidermal cell of considerable size, the meaning of which is obscure. It 

 has been suggested that it represents a degenerated, obliterated enteron in 



FIG. X. 



Four stages in the development of the male of Rhopalura (after Julin). In A the maeromere 

 has become partially invested by the micromeres (a), and has already divided, giving rise to the 

 primary germ cell (6), and to an intermediate cell (c). In B the maeromere has again divided, 

 giving off an anterior intermediate cell (c'). In C the central cells have become entirely sur- 

 rounded by the ectoderm (), which is ciliated. The primary germ cell has subdivided and 

 now constitutes the testis. In D the ectoderm cells have become differentiated, in that the 

 cilia of the anterior cone are directed forwards, the rest backwards ; the testis is assuming the 

 condition of the adult, though the intermediate cells have not yet given rise to muscle fibres. 



other words, it is hypoblast ; but there appears to be no evidence in support 

 of this view. 



In both females the central mass consists of egg cells ; these are 

 free in the cylindrical form, and are discharged by dehiscence of the 

 ectoderm at the level of the non-ciliated ring. They are fertilised by 

 spermatozoa, and develop into males. 



The eggs of the flattened form are embedded in a granular mass, and 

 develop parthenogenetically into females of either form. 



The females occur in a peculiar vacuolated mass of granular material 

 in which no cellular structure is to be detected. This " plasmodial tube " 

 (Metschnikoff, or " sporocyst " of Giard) fills up the body cavity of the 

 host, but is enveloped in a nucleated layer, which appears to be part of 

 the tissues of the host. Metschnikoff believed that both sexes passed 

 their entire life in these " plasmodial tubes," but Julin has never detected 

 males or embryos of males in them. These only occur free in the fluid of 

 the body cavity. 



