I 5 6 THE ORTHONECTIDA 



These plasmodial tubes appear to be formed in this way : The flattened 

 females fragment ; each piece, containing a number of eggs, rotates for a 

 time in the cavity of the host, but later becomes attached to the wall of 

 this cavity ; the ectoderm cells now drop off, swell up, and undergo various 

 changes resulting in the formation of the plasmodium. On the other 

 hand, it is suggested by Max Braun that the latter arises by degeneration 

 of the gonads of Amphiura, owing to the attacks of the parasite. 



The development of the fertilised egg leads to a blastosphere consist- 

 ing of a central macromere surrounded by ectoderm cells (Fig. X.) ; the 

 former divides, twice, unequally producing a small cell at each end, whence 

 the " intermediate cells " arise (cf. the primary germ cells of Dicyemida), 

 the macromere now divides up into numerous germ cells. There thus 

 appears to be no remains of an endoderm in this history ; the macromere 

 is mesoblastic, giving rise to muscular cells, and genital cells. In the 

 history of the parthenogenetic eggs, the homologue of the terminal 

 "intermediate cells" appears to be a continuous layer between the 

 ectoderm and the axial cell, so that the two forms of females can be 

 recognised at a very early stage. 



[Caullery and Mesnil in the last three years (1898-1900) have described 

 four new species of Orthonectids : (1) Ehopalura metschnikovi, parasitic 

 both in a Chaetopod (Spio martinensis, Mes.), and a Nemertine (Tetra- 

 stemma flavidum). (2) Eh. julini, parasitic in a Chaetopod (Scotelepis 

 fuliginosa, Clpde.). (3) Rh. pelseneeri, parasitic in Tetrastemma flavidum. 

 (4) Staecharthrum giardi, parasitic in a Chaetopod (Scotoplos muelleri, 

 Rathke). The two first-named species present distinct males and females 

 as do the previously known species of Rhopalura. On the other hand, 

 Rh. pelseneeri is hermaphrodite, as is also Staecharthrum giardi, which 

 differs from all other Orthonectids by the great length of its filiform body. 

 According to the observations of these authors (Comptes rendus Acad. 

 Sciences, 20 fe"vrier 1899, and later results privately communicated), 

 which have extended to Rh. ophiocomae, Giard, as well as the species above 

 named, the plasmodiurn is a true protoplasmic nucleated structure, 

 capable of amoeboid movement. It is under this form that the Ortho- 

 nectid makes its first appearance in the infected host. By segregation of 

 certain of the nuclei and portions of surrounding protoplasm of the 

 plasmodium, germ-cells are produced, and from these develop ciliated 

 embryos which finally become adult males and females. The same 

 plasmodium can give rise to both males and females. The sexual 

 products of these ciliated offspring of the plasmodium are not discharged 

 within the host, but only after the escape of the males and females into the 

 sea-water. The sexually fertilised egg-cell and the resultant embryo are 

 unknown, but it is this embryo which effects an entry into a new host 

 and becomes a nucleated plasmodium which in turn again produces 

 ciliated males and females. Thus there is an alternation of genera- 

 tions, sexual and asexual. The comparison of the plasmodium to the 

 sporocysts of Malacotylous Trematoda by Giard appears to be justified, 

 in so far that both are the first forms assumed by the sexually produced 

 individual on entering upon parasitic life ; and both give rise asexual ly 



