154 



THE ORTHONECTIDA 



R. giardii has been the subject of Julin's 1 researches, and is of more 

 frequent occurrence than R. intoshii. The present account chiefly refers 

 to the former species. The male is minute and spindle-shaped (Fig. VII.). 

 The ectodermal rings are six in number, including the anterior and 

 posterior terminal cones. The whole body, with the exception of the 

 second ring, is ciliated. The cilia borne by the anterior cone are 

 throughout the Orthonectida directed forwards, but elsewhere they point 

 backwards. The third ring is larger than the others, but the second ring 

 is noticeable for the refringent granular knobs borne by the cells. This 

 ciliated ectoderm encloses the " testis," a mass of cells enveloped in a 

 distinct membrane. In both sexes this central mass of cells, which give 



FIG. VII. 



Rhopalura, male (after 

 Julin). 



FIG. VIII. 



The cylindrical female of Rho- 

 palura (after Julin). 



FIG. IX. 



The flattened female of 

 Rhopalura (after Julin). x, 

 the peculiar subepidermic 

 cell. 



rise either to spermatozoa or to ova, can be traced back, in the embryo, 

 to a single axial cell, which, as in the Dicyemida, is the larger of the first 

 two blastomeres into which the egg cell segments (Fig. X.). But whereas 

 in Dicyemida the genital cells are derived from a " primary germ cell " 

 at each end of the axial cell, here the axial cell itself subdivides to pro- 

 duce germ cells, which represent, in all probability, mesoblast, whilst the 

 last trace of " endoderm " has gone. Between the " testis " and the ecto- 

 derm a number of fine, obliquely directed streaks can be detected, forming 

 a continuous sheath (Julin), or limited to four bundles (Metschnikoff). 

 The ends of these fibrillae, which are regarded by Julin as muscular, 

 appear to be continuous with ectoderm cells at each end of the body. 

 Julin figures nuclei on them, superficial to the testis. Possibly they are 

 1 Julin, Arch. BioL iii. 1882, p. 1. 



