140 OLIGOCHAETA 



maturity and, in consequence, the development of the clitellum. I kept under 

 continual observation a quantity of Aeolosoma tenebrarum for more than a year 

 without once seeing the clitellum or sexual organs. 



Among the terrestrial Oligochaeta, on the other hand, particularly among the 

 exotic species, the clitellum seems to be more or less permanent, after they have 

 reached their full size. It is quite possible that this permanency belongs to the 

 species rather than to the individual ; but in any case it will be noted, on reference 

 to the systematic pages of this work, how very few terrestrial Oligochaeta there 

 are in which the clitellum is unknown. Considering that in so many cases the 

 species are known from an examination of very few examples, collected at all times 

 of the year, the assertion with which I have commenced this paragraph seems to 

 be, to some extent at any rate, justified. 



The clitellum is a modified tract of epidermis, whose minute structure has 

 been already dealt with. It generally happens that the muscular layers in this 

 region of the body have undergone some diminution in thickness, in spite of which, 

 however, the clitellum generally stands out above the general body surface. Its 

 colour, too, is as a rule different from the rest of the body ; but not always. In the 

 transparent aquatic Oligochaeta the clitellum, on account of its thickness, and on 

 account of the enclosed genital products, is of an opaque white. As to minute structure 

 the clitellum shows two principal modifications: in the aquatic Oligochaeta, without 

 exception, and in the genus Moniligaster there is only a single layer of glandular 

 cells, the thickness of the clitellar epidermis being thus but little in excess of that of 

 the general body-surface. In all other Oligochaeta, whose minute structure has been 

 investigated, the clitellar epithelium is composed of several layers of cells. This 

 important morphological difference is not at all due to size. Microdrilus and 

 Microscolex, besides a quantity of other terrestrial species, are no larger than 

 Moniligaster bahamensis, and not so large as Phreoryctes smithii. The difference 

 in the structure of the clitellum may possibly be physiological ; the thinner layer 

 of epidermis might secrete a thinner cocoon ; and a cocoon deposited in water or 

 watery mud would run less risks of having its contents dried up than one deposited 

 near the surface of the soil. As, however, Moniligaster seems to be a purely 

 terrestrial genus, this explanation cannot yet be accepted. In the meantime, therefore, 

 I would regard the difference in the histological structure of the clitellum as 

 a morphological difference, acquired so remotely that it cannot at present be 

 explained by environmental or other physiological causes. 



The clitellum of the Oligochaeta in naked-eye anatomical characters shows three 

 principal variations. 



