On the .striu'turn of tlio Mc|ilirifii'i of Stidiostcmma. 207 



Accordingly, we find the length of these organs to vary from 

 27 // to 1422 // ; and it may be observed that on the left side, where 

 the fewest occur, the longest uephridium is found. 



The following parts constitute the individual nephridia: 1) the 

 internal teriuinalions of the organ, to which Bijugek ('92) has applied 

 the term "Kölbchen"', and which I shall call the terminal bulbs; 

 2) the fine canals connecting the former with the main ducts of the 

 organ, which may be referred to as the uephridial ductules; 3) the 

 main ducts, and 4) the excretory ducts. The histology of these parts 

 shall be spoken of below, but the arrangement of 3) and 4) remains 

 to be described. 



All these parts lie within the body-cavity, between the longitudinal 

 muscle layer of the body-wall, and that sheath of connective tissue 

 which envelops the lateral nerve-chords and gonads, and covers the 

 intestine. 



In the trunk region of the body the main longitudinal ducts lie, 

 on each side, in the dorso-lateral segment of the body, either close 

 to the rhynchocoelic sheath, or somewhere between the latter and the 

 nerve-chord ; thus their course is not straight, but more or less sinuous. 

 These main ducts are, however, not in the whole extent of their 

 course single canals, but at frequent points there are formed masses 

 of inextricable loops and circumvolutions, which bear quite a striking 

 resemblance to an Annelidan nephridium ; such a mass of circumvoluted 

 ducts is to be seen in Fig. 8 Nepli. The ducts situated above tjie 

 nerve-chord may be termed dorsal ducts, in order to distinguish them 

 from those portions below the chords, for which the application ventral 

 ducts seems fitting. These latter are branches of the former, and at 

 least one, in the longer nephridia two or three, or even more ventral 

 ducts are given off from the dorsal ones. Again, a dorsal duct without 

 branching may pass below the nerve-chord ; so that it is somewhat 

 difficult on a study of consecutive sections, to determine whether a 

 duct found below the nerve-chord is a true ventral duct (i. e. a branch 

 of the dorsal), or whether it is a portion of the dorsal duct, which 

 has at one point passed beneath the nerve-chord. In the most anterior 

 and most posterior portions of the body, the main nephridial ducts 

 are placed altogether above the nerve-chords. On opportune sections 

 one finds the ducts situated in the main body -cavity, everywhere 

 except in the dorsal and ventral median line. 



In the same large individual, I found on the right side 19 ex- 

 cretory ducts, of which 15 were placeil above the nerve-chord, and 



