I have recently (1941, p. 30) referred to the small differences which separate the two genera Megascolides McCoy and Notoscolex Fletcher, the only fundamental difference being the microscopic structure of the prostate.
So long ago as 1892, Beddard (p. 130) wrote: “The discrimination of the genera of the Cryptodrilidae is unfortunately the most difficult part of the classification of the Oligochaeta.” And so it is to-day, 50 years later, as the number of species has been studied anatomically.
As an illustration of this uncertainty, mention may be made of my genus Tokea (1905). In 1907 Michaelsen (p. 161) regarded this genus as really included in Megascolides. In 1916 (according to Stephenson (1923, p. 193, and 1930, p. 835), for I have not access to the original paper, Michaelsen includes it in the genus Notoscolex on the ground of the microscopic structure of the prostate. Again, in 1910, Michaelsen (p. 36), in discussing the geographical distribution of these and related genera, refers repeatedly to the occurrence of Megascolides in New Zealand and Ceylon, but in 1916 comes to the conclusion that New Zealand earthworms formerly attributed to Megascolides must be removed from that genus to Notoscolex. Consequently New Zealand must be excluded from the geographical distribution of Megascolides. And so my Tokea has been shifted about, first as a subgenus of Megascolides in 1907 (p. 161), and then to Notoscolex in 1916. Now when our greatest authority on the Oligochaeta (unfortunately deceased) is so uncertain as to the limitation of the two genera, refers repeatedly to the occurrence, it is not surprising that Stephenson (who is likewise defunct), and whc was, as his Monograph reveals, the next authority on the group, but who had followed Michaelsen, should have presented two opinions as to Tokea, for in 1930, p. 658, he refers to the edibility of “Tokea (Megascolides),” while on p. 837 he wrote: “Tokea must now go into Notoscolex.” Confusion worse confounded!
As I have mentioned above, the fundamental difference between the two genera lies in the microscopic structure of the prostate, but many species have been described attributable to one or other of these two genera, in which, however, the internal structure of this organ is unknown; and Stephenson (1923, p. 194) remarks: “But to reduce the necessity of resorting to this procedure, it may be assumed that the flattened, tongue-shaped gland, especially if the boundaries have any trace of lobing, will have branched ducts; while glands which are definitely cylindrical in shape will quite possibly have a simple duct.” This seems rather like guesswork, and can only be employed if the author of a species has described or figured sufficiently carefully the form and appearance of the prostate, and unfortunately this has not always been the case. But this illustrates the awful state of uncertainty that surrounds any attempt to distinguish these and some allied genera.
I have alluded to Stephenson's views in my former paper (p. 31). I have allotted the present worm to the genus Notoscolex rather than to Megascolex or to Tokea on the following grounds: There are only three pairs of spermathecae, and the prostate is tongue-shaped with lateral incisions and lobings along its margins, which suggest a branching system of canals. It differs in this respect from Megascolides, while from Tokea it differs in (a) the external and therefore probably the structure of the prostate; (b) the absence of recognisable diverticula on the spermathecal duct; (c) the gizzard is in VI instead of V; (d) absence of meganephridia in the posterior segments.