pair on each side. In describing the arrangement of chaetae, those of each side are designated in order from the ventro-medial to the dorso-medial by the letters a, b, c, d. This is the most frequently occurring chaetal arrangement and from it, in many genera, has been derived the other arrangement.

(b) Perichaetine—as found in Plagiochaeta, where there is a large number of chaetae, or pairs of chaetae evenly spaced around each segment.

The lumbricine arrangement of chaetae is invariable, but in those genera which have the perichaetine arrangement, there may be variations between species in the number of chaetae, or pairs of chaetae, on each segment. In such cases the number of chaetae per segment is often a valuable specific character (as in some species of Pheretima).

(2) Nephridia: The usual condition is the meganephridial condition, in which there is in each segment a pair of elongate coiled nephridial tubules opening to the exterior by a pair of small pores. From the meganephridial condition, in many genera has been evolved the micronephridial condition, as seen in Octochaetus, where there is a large number of small nephridial tubules usually arranged in a band round the lateral aspects of the peritoneum or in paired clusters, one on each side of the ventral nerve cord in each segment. A condition intermediate between the meganephridial and the micronephridial conditions is seen in some genera, e.g. in certain species of Megascolides there are micronephridia in each segment, but in some of the posterior segments meganephridia occur in association with them.

(3) Arrangement of the Nephridiopores: In nearly all the meganephridial genera of the Megascolecidae the nephridiopores are in a single series on each side of the body, but in the three genera comprising the Neodrilacea (a group of genera peculiar to New Zealand) and in some species of Perionyx the nephridiopores alternate more or less regularly between a dorso-lateral and a ventro-lateral position in successive segments.

(4) Male Genital Apparatus: The number and positions of the prostatic pores and of the associated male pores are used to distinguish genera. The more general arrangement is the “acanthodriline,” in which there are two pairs of prostatic pores, a pair on segment xvii and a pair on segment xix, and a single pair of male pores on segment xviii (as in Acanthodrilus). The “acanthodriline” arrangement may be considered as the primitive arrangement and from it the other two arrangements may be derived in the following manner:

(a) The “microscolecine” arrangement in which the posterior pair of prostates and their pores disappear and the male pores may, or may not, unite with the prostatic pores on segment xvii or on segment xviii (e.g. Rhododrilus, Megascolides).

(b) The “balantine” arrangement in which the anterior pair of prostates and their pores disappear, and the male pores unite with the prostatic pores on segment xviii (e.g. the South African genus Udeina).

These arrangements may undergo variations, and the importance attached to such variations is not standardised. Benham (1909) distinguished the genus Leptodrilus for two species which resembled