CLASS ROTIFER A. 189 



manner iu which the ova make their escape is yet unknown. 

 Both ovaries and oviducts are enclosed within a fold of peri- 

 toneum (mesentery) extending from the sides of the body. 

 The testes (t) are situated behind the posterior dissepiment, 

 i.e., in the tail region of the body, and consist of a streak of 

 cells on each side in the peritoneal covering of the body-wall. 

 From these streaks masses of immature spermatozoa separate 

 and float about in the coelorn of the tail segment, and when 

 mature make their escape through canals, each of which com- 

 municates with the coelorn by means of a fine ciliated opening, 

 and near its opening to the exterior at the side of the body is 

 dilated into a seminal vesicle. 



The embryological history of Sagitta throws no light upon the affinities 

 of these forms. In structure they recall, especially in the arrangement of 

 their musculature, the Nematodes, and especially the Gordiaceae, but at the 

 same time show many similarities to the lower marine Annelids, as for in- 

 stance in the origin of the spermatozoa from the wall of the coelom, and in 

 the similarity of the vasa deferentia to nephridial canals. The occurrence 

 of dissepiments also suggests affinities to the Annelids, but it does not 

 seem that these structures indicate a segmentation of the body, since the 

 arrangement of the nervous system points to the conclusion that the Chae- 

 tognaths consist of a single segment. From the evidence at present open 

 to us it would seem that the Chcctognatha are more nearly related to the 

 Annelids than to the Nematodes, but the relationship must be regarded as 

 a rather remote one, and it seems hardly fitting to include Sagitta and its 

 allies among the Annelida. 



CLASS ROTIFEEA. 



. 



The Rotifers or " Wheel-animalcules " are microscopic 

 Metazoa which are widely distributed both in salt and fresh 

 water. They are unsegmented forms with a well-developed 

 coslom, and are somewhat oval in form as a rule, the an- 

 terior end of the body being surrounded by one or two 

 bands of cilia whose rapid movement produces the appear- 

 ance of a wheel, and has suggested the popular name for 

 the group. The posterior end of the body is frequently pro- 

 longed into a usually extensible so-called foot, which in some 

 cases (Lacinularia) is furnished with adhesive glands, and is 

 used as a point of fixation, though the majority of forms swim 

 about freely or attach themselves only temporarily, the foot 



