DESCRIPTIONS OF GENERA AND SPECIES 



455 



Cryptodrilidae, including the two genera here treated of, in the fact that the two 

 paii-s of spermiducal glands lie not in successive segments, but are separated by a 

 segment which bears the apertures of the sperm-ducts. It is very possible, however, 

 that this genus, Kerria, is a stage in the development of the genus Ocnerodrilus out 

 of AcanthodnLus, or it may be that the resemblance is due to a convergence caused 

 by a simplification in structure; in this case it might be said that the likeness 

 between Ocnerodrilus and Gordiodrilus is something of the same kind; but here 

 the resemblance is rather closer; indeed, the only difference between the two genera 

 is that the structure of the calciferous glands is a little difierent, and that the male 

 pore opens upon a different segment, independently also of spermiducal glands. 



Besides the genera just referred to which appear to be simpler in structure 

 than the more typical forms, there are three other genera not nearly related to 

 those just described, but also showing some evidence of degeneration; these are 

 Pontodrilus, Photodrilus, and Microscolex; the latter, however, is not quite so 

 simplified as are the first two genera. 



They are all three of small size ; there is often no gizzard, or merely traces of 

 a gizzard ; there are sometimes no dorsal pores ; the nephridia do not commence 

 until the fourteenth or the fifteenth segment, except in Microscolex, where they begin 

 earlier, and they are paired structures ; the subnervian vessel is absent ; there is no 

 typhlosole. 



In these particulars the three genera appear to be somewhat degenerate as 

 compared with other Cryptodrilidae, though not so much so as are the genera 

 Gordiodrilus and Ocnerodrilus ; in this case, as in that, smallness of size accom- 

 panies simplification of structure. 



The following table indicates the mutual relationships of the three : — 



Setae . 

 Male pores . . 

 Nephridia . . . 

 Gizzard . . . 

 Calciferous glands 

 Last heart 

 Spermathecae . 

 Spermiducal glands 



miCEOSCOLEX. 



in eight x-ows, or paired 



xvii 



begin in ii-v 



present, rudimentary, or absent 



absent 



in xii 



one pair 



tubular 



PONTODKILUS. 



in eight rows 



xviii 



begin in xv 



rudimentary 



absent 



in xiii 



two pairs 



tubular 



PHOTODKILUS. 



in eight rows 



xviii 



begin in xiv 



absent 



one pair 

 tubular 



