482 EDWIN S. GOODRICH. 



About the Cestodes and Trematodes it need only be said that 

 they are built (so far as concerns the question discussed in this 

 paper) upon essentially the same plan as the Planarians;^ 

 while as to tlie Nematodes, of the development of which we 

 know too little, it seems probable that here also the genital 

 follicles represent the coslom, while the body cavity is a blood- 

 space corresponding in its relations to the parenchyma of the 

 Planariaus. 



ROTIFERA. 



The Rotifers, recently described in great detail by Platte (87, 

 88) and Zelinka (115), agree in the general structure of the 

 nephridia, genital follicles, and genital ducts, so closely with the 

 Platyhelminths that they may be dismissed with a very few 

 words. The nephridia are a pair of branching tubes ending 

 internally in flame cells, and opening behind into the cloaca ? 

 The coelom is represented by a pair of genital follicles, 

 one of which only is generally developed; the wall of the 

 follicle is produced backwards to form the genital duct, or 

 peritoneal funnel opening into the cloaca. 



The development of the nephridia has not yet been thoroughly 

 worked out. Zelinka has traced them to a group of cells of 

 doubtful origin . He adds : '^das Exkretionssystem konnte ich in 

 so fern mit Sicherheit auf das Ektoderm zuriickfiihren, als es 

 bestimmt nicht auf das Entoderm bezogen werden kann '' (115). 

 until tlie mesoblast has been formed, and, generally speaking, that the greater 

 the development of mesoblast the more definite is the vascular system. I 

 should rather consider the connection of the blood-spaces with the blastoccel 

 as arising for purely mechanical reasons, so to speak, and in no way of pliylo- 

 genetic significance. The temporary continuity of the blastoccel with the 

 blood-space during the ontogeny of certain of the higher forms would seem to 

 be connected with that method of development by the folding of germ-layers 

 or sheets of tissue, which no one would look upon as primitive. During this 

 process, the cavities which will form the vascular system later on, are ine- 

 vitably continuous with the space left between the germ-layers. 



1 Fraipont maintains that the flame end-cells of the nephridia of the Cestodes 

 and Trematodes communicate internally by means of a small lateral aperture 

 (39). 



2 As in the case of some Platyhelminths, the nephridia have been stated to 

 open internally (Eckstein, 28). 



