igoS.] J. Stephenson: The Anatomy of some Aquatic Oligochceta. 259 



is, however, variable. Usually the stomach is fairly sharply delimited anteriorly 

 (fig. i), the œsophagus in some cases being invaginated backwards into it after the 

 manner of an intussusception ; the stomach is less sharply delimited posteriorly, 

 where it is continued into the ntestine; it may appear as merely a gentle fusiform 

 dilatation on the alimentary tube ; or in some cases may not be distinguishable at all. 

 In the latter case, there is then no differentiation of the alimentary canal behind 

 the pharynx. The intestine is ciliated, the cilia being obvious, and working in a 

 postero-anterior direction ; peristaltic movements throughout its extent occur con- 

 stantly and with a fairly regular rhythm; these movements, like the ciliary action, 

 proceed from behind forwards, and, borrowing a term from mammalian physiology, 

 may for convenience be described as '^antiperistaltic." It would seem, therefore, 

 that the intestine performs a respiratory function. 



Circulatory system. — The blood is yellowish red, and contains no corpuscles. 

 The dorsal vessel is contractile, the contractions progressing in a postero-anterior 

 direction ; it is incorporated in the wall of the alimentary canal as far forwards as 

 the oesophagus ; the brown globules present in the wall of the intestine may be seen 

 superficial to the vessel. The ventral vessel is non-contractile and is not incorporated 

 in the intestinal wall ; it divides about the level of the setal bundle of the third seg- 

 ment, and the branches join in the prostomium to form the dorsal vessel ; for the rela- 

 tions of the blood vessels to the nerve ganglion vide fig. 15. There are not fewer than 

 four, perhaps five, transverse commissures joining dorsal and ventral vessels in the 

 pharyngeal region ; they occur in the third, fourth and fifth segments, but apparently 

 there may be more than one in a segment. 



Nephridia.— The first nephridium is in the seventh segment. The beginning 

 of the tube can be seen as a ciliated open mouth, or small ciliated funnel, in 

 the preceding segment; the tube at once pierces the septum, and is then somewhat 

 dilated for a short distance, after which it appears to maintain a uniform diameter 

 throughout its numerous windings till it opens into a terminal ciliated dilatation 

 which discharges to the exterior on a level a little in front of the insertion of the 

 ventral setae. The walls are composed of a granular protoplasm in which nuclei 

 and cell outlines are not to be distinguished in the living animal. 



Nervous system. — The cerebral ganglion is deeply indented behind, less deeply 

 in front (fig. 15). The commissures join its anterior part (figs 2 and 15); the first 

 ganglion of the ventral chain is immediately behind the mouth ; the ganglia in general 

 can be seen in a side view as swellings, situated each at the level of the ventral setse ; 

 seen from the ventral surface the cord has an irregular lobulated outline, and the 

 ganglionic swellings are not distinguishable from this aspect. 



Genital organs. — The genital organs were observed in various stages of develop- 

 ment in the months of March and April. The appearances were as follow : as to the 

 interpretation of the appearances I am not in all cases quite clear. Both sexual and 

 asexual reproduction may go on together ; an individual that was about to divide 

 asexually was not uncommonly found to have sexual organs in a moderately advanced 

 stage of development. 



