COMMON EARTHWORM. 305 



terrestrial and most of the aquatic Oligochaeta and the Leeches, and is probably 

 therefore derived from an invagination of the hypodermis, as is the vesicle of the 

 aquatic forms, the rest of the nephridium being mesoblastic. The pore by which 

 the duct opens is generally placed slightly in front of the setae in the ventral row, 

 but it may be placed in a similar position relative to the dorsal row. There seems 

 to be much variability in the position of the aperture in the same individual, and 

 the apertures of the right and left organs in the same somite may not correspond 

 (cf. Claparede, Z. W. Z. xix. 1869, p. 620; Hering, Z. W. Z. viii. 1856-57, note, 

 p. 401). The variation is perhaps due to the loss of nephridia originally present 

 in each somite, and opening in different positions relatively to the setae. Perrier 

 observed in Plutellus an alternation in the position of the nephridial apertures 

 between the upper and lower rows of setae. Beddard has noted a similar but 

 irregular alternation in a species of Acanthodrilus in which the dorsally placed 

 nephridia have the duct represented by a large thin-walled muscular sac, with a 

 small diverticulum ; whereas those ventrally placed open laterally into a long 

 muscular sac, much as do the nephridia of aquatic Oligochaeta. There is there- 

 fore an anatomical difference between the two sets of nephridia opening dorsally 

 and ventrally. The last-named anatomist has also described the nephridia of a 

 New Zealand Acanthodrilus (A. multiporus), in which there are four nephridia on 

 each side in each somite of the body, corresponding to the setae. The two 

 ventral nephridia on each side are intertwined and anastomose (?). In the pos- 

 terior region of the body the organs open externally, each by a single aperture 

 placed dorsally to each of the eight separate setae ; in the anterior region, however, 

 there are innumerable minute orifices in a single circular row, and in the same line 

 with the setae, due to the branching of the ducts which appear to form a circular 

 canal round the somites in this region. It is quite possible therefore that in other 

 Earthworms certain of the nephridia in each somite are aborted, but that it is 

 sometimes one, sometimes another pair which is missing. Benham states (Q. J. M. 

 xxvi. p. 256) that numerous small nephridia occur in each of the somites of a small 

 Perichaeta from the Philippine Islands. Details are wanting at present. 



The aquatic Oligochaeta have a single pair of nephridia in each somite opening 

 near the ventral set of setae, and composed of the same parts as in Lumbricus. 

 The duct, which is sometimes glandular, is usually vesicular, and the gland-tube 

 opens into it laterally. The funnel is absent in Chaetogastridae. Vejdovsky finds 

 that both funnel and gland-tube are produced by the growth of peritoneal cells 

 covering the septa : the duct by an invagination of the hypodermis. 



Nephridia of the ordinary shape appear to be absent in Megascolex and 

 Typhaeus, and their place is taken by tufted bunches of tubes, the structure of 

 which has not been investigated. But they appear to be closely similar to the 

 first pair of nephridia of Urochaeta, and the nephridia in general of Microchaeta. 

 Each organ in the last named consists of a muscular vesicle opening externally to 

 which are attached (i)a series of U-shaped loops, each loop containing intra-cellular 

 tubes, and (2) a single simple tube which perforates a septum in the usual manner 

 and opens into the coelome by a small contracted funnel. The mode of con- 

 nection of the tubes to the vesicle has not been ascertained as yet. 



The Earthworm is hermaphrodite like most of its congeners. The organs 

 are figured in PI. xii. The male apparatus consists of two pairs of testes, three 



