THE ANATOMY OF SUTROA. 197 



spoken of as loosely surrounding the commencement of the atrium, allows plenty of room 

 for the subsequent development of additional prostates. The prostates, as Eisen says, 

 are composed of masses of pear-shaped cells ; the ducts of which (a prolongation of each 

 cell forms its duct) open into the lumen of the atrium. 



The efferent apparatus of this worm is evidently very interesting ; it is constituted 

 upon the plan of the Lumbriculidae ; but there are differences from the typical Lumbri- 

 culidae. The chief difference is in the structure of the atria. In all the Lumbriculidae 

 hitherto known the atria are globular sacs with a specially thickened peritoneal layer — 

 occasionally termed " prostate." The same kind of prostate occurs in a genus of 

 Tubificidae recently described by myself under the name of Branchiura* and also in 

 the genus Moniligaster.i In both these genera the large pear-shaped cells which clothe 

 the atrium externally do not communicate with the lumen of the atrium ; the prolonga- 

 tions of the cells do not perforate the muscular layer which separates them from the 

 epithelial lining of the atrium. Nor do they, according to Vejdovsky's figures, in 

 Rhynchelmis.\ On the other hand, in the Tubificidae the " Cement-driisen " are masses 

 of cells which look at first sight very much like the glandular investing cells of the 

 Lumbriculidae, but are really outgrowths of the epithelium of the atrium. This has been 

 proved developmentally. 



The origin of the prostates of Sutroa has yet to be studied ; but in the meantime 

 they suggest those of the Tubificidae more than those of other Lumbriculidae. 



The next question is, What is the nature of the membranous sac surrounding the 

 atria and the prostates ? It should be mentioned as a preliminary that this sac also 

 surrounds the sperm-sacs. I think that in all probability this delicate membranous sac 

 is the proper wall of the sperm-sac. The other organs only happen to lie within it, just 

 as the testes lie within the sperm-sacs among earthworms. 



A year ago I communicated to this Society a paper upon a remarkable new genus 

 of Oligochaata, which I named Phreodrilus; in this worm the atria and the vasa 

 deferentia were surrounded by a membranous sac§ which appeared to be merely the 

 peritoneal layer of the atrium and the vas deferens separated from the subjacent layer. 

 The sac thus formed contained spermatozoa ; I compared this arrangement to something 

 of the same kind described by Eisen in Eclipidrilus. It may be that here too we have 

 a sperm-sac surrounding the atrium and the vas deferens ; but while in Phreodrilus the 

 sac in question is nothing more nor less than the peritoneum stripped off from the atrium, 

 in Sutroa a layer of peritoneum remains behind. 



The study of the development can alone tell us whether there is in Sutroa an 

 actual splitting of the peritoneum, or whether there is a formation of a separate sac 

 comparable to the sperm -sac of other Oligochaeta. 



* On the Anatomy of a new Branchiate Oligocha^te (Branchiura Sowerbii), Quart. Joum. Micr. Sci., vol. xxxiii. 

 t On-some Earthworms from the Philippine Islands, Ann. and Mag. Nat. Hist., Feb. 1886. 

 X Anatomische Studien an Rhynchelmis limosella, Zeitschr. f. iviss. Zool., bd. xxvii. 



§ Anatomical Description of two new Genera of Aquatic OligochEeta, Trans. Roxj. Soc. Edin., vol. xxxvi. part ii. 

 No. 2. 



