MEGADRILI 383 



all the species of the genus Gordiodrilus. This family is one 

 which it is exceedingly difficult to define and to split up into 

 different genera. It shades almost imperceptibly into the Peri- 

 chaetidae on the one hand, and is very hard to differentiate from 

 the Acanthodrilidae on the other. A Cryptodrilid, like any 

 member of the genus Cryptodrilus, with complete circles of chaetae 

 would be a Perichaetid ; and as there are species of Perichaeta in 

 which the anterior segments have only a few chaetae in each 

 segment, it is perhaps wrong to separate the two families at all. 

 Apart from the chaetae, there is no peculiarity in the organisation 

 of the family Perichaetidae that is not also met with in the 

 Cryptodrilidae. Even the highly characteristic intestinal caeca 

 so distinctive of the genus Perichaeta itself, as contrasted with 

 Megascolex and the other genera, occur, though more numerously, 

 in the African Millsonia, where there are forty or fifty pairs of 

 them. A fairly common feature in the family is the presence of 

 two, or even three, pairs of gizzards, a character which is also 

 met with in the genus Benliamia among the Acanthodrilidae, 

 and occurs also in some other families. The names Digaster, 

 Didymogaster, Perissogaster, and Dichogaster have been founded 

 upon this character. The excretory organs may be paired (in 

 Trinephrus there are three pairs to each segment) or of the diffuse 

 kind. The male pores are usually upon the eighteenth segment, 

 but not unfrequently upon the seventeenth, and are often armed 

 with long and ornamented chaetae. Spermiducal glands are 

 invariably present, and may be lobate or tubular. There are 

 two groups of small-sized genera, which in their simplicity of 

 organisation stand at the base of the series ; but it is very 

 possible that the simplification is rather due to degeneration 

 than to primitive position. One of these groups includes the 

 semi -marine genus Pontodrilus (with which I include the 

 phosphorescent Photodrilus) and Microscolex. In these forms 

 the gizzard has disappeared, or is represented by a rudimentary 

 structure, and the male pores are upon the seventeenth segment. 

 In the other group are the genera Ocnerodrilus, 1 Gordiodrilus, 2 and 

 Nannodrilus, which are of even smaller size, and have in the same 

 way the male pores upon the seventeenth segment. The species 

 of this group are often aquatic, and there is not only no gizzard 



1 Eisen, " Anat. Studies on Ocnerodrilus" Proc. Calif. Acad. (2) iii. 1892, p. 228. 

 2 Beddard, Ann. Mag. Nat. Hist. (6) x. 1892, p. 74. 



