ON TUBIFEX niVULORUM. 167 



vesicle, is further described in connection with the vas deferens. 

 It is a large pouch, grafted to the atrium in a very remarkable 

 manner. Its wall is continuous with the two internal layers of the 

 atrium, but not with the external layer ; the latter, therefore, 

 presents a perforation through which the communicating neck 

 between the seminal vesicle and the atrium passes. The edges 

 of this perforation are thickened, and the appearance is thereby 

 produced of a ring encircling the neck of the vesicle, as a graft on 

 a tree is encircled by the bark. The vesicle itself is tilled with 

 epithelial cells. 



The female organs are enumerated as follows : — An ovary, an 

 oviduct, and a pair of seminal receptacles (see Fig. i). The ovary 

 is double, and placed in the eleventh segment. Each ovary is 

 pear-shaped, and adheres by its narrow extremity to the posterior 

 surface of the septum separating the tenth from the eleventh 

 segment, and is formed by the agglomeration of a multitude of 

 ovules. The eggs arrive successively at maturity, and having 

 attained twenty or thirty times their original volume, are pressed 

 against the septum eleven — twelve, whence they pass alongside 

 the second testis, and are speedily followed by others. The septal 

 sac in which the testis and eggs thus lie, and which we may regard 

 as a matrix, sometimes extends as far as the seventeenth segment, 

 but it is open in the eleventh, and the mature eggs must thus be 

 considered to float in the perivisceral cavity. The opening by which 

 the eggs arrive at the exterior is difficult to find in Tiibifex^ and 

 Claparede infers, in the absence of direct observation, that the 

 passage taken by them lies between the thick outer wall of the 

 atrium, and the inner membranous one. It will be remembered 

 that the former is described as perforate around the neck of the 

 seminal vesicle, and it is thought that the thickened ring forming 

 this perforation, opens and allows the eggs to pass. The atrium, 

 or lower portion of the vas deferens would, according to this view, 

 be invaginated in the oviduct; the orifices of these two organs 

 are described as situated one within the other in the sexual orifice, 

 which is found in the eleventh segment. Lastly, there is a pair 

 of sac-like seminal receptacles situated and opening externally into 

 the tenth segment, in which Claparede remarks that there is 

 sometimes found an Opalinoid parasite belonging to the genus 

 Fachydcrmon. 



