DESCRIPTIONS OF GENERA AND SPECIES 511 



the seventeenth segment ; indeed, every possible variation occurs. This segment is the 

 one which bears the male pores. In the type-species of the genus, 0. occidentalis, 

 and in one other, 0. limicola, both setae of the pair are present and unchanged ; this 

 may be fairly regarded as the primitive condition. The next stage is shown in 

 those species in which one or other of the two setae have disappeared. Finally, the 

 majority of the species (five) are entirely without the ventral pairs of setae. 



EiSEN considers that the genus is naturally divisible into two groups; in one 

 of these the lower end of the sperm-duct is enveloped by a thickish muscular 

 sheath ; in the other there is no such sheath. The former state of affairs characterizes 

 these species ; the character is a peculiar one, and is met with in other worms 

 belonging to different families ; it may be noted that all the species, viz. 0. agricola, 

 0. rosae, and 0. contrcmtus, which present this structural peculiarity, have no ventral 

 setae upon the seventeenth segment. 



0. limicola is alone in possessing two pairs of spermiducal glands. Though in 

 this character the species in question agrees with the nearly related genus Oordiodrilus, 

 it must not be forgotten that an important difference between the two is the fact that 

 in Ocnerodrilus the sperm-ducts open in common with the spermiducal gland and on 

 to the seventeenth segment. 



The spermathecae (of which there is never more than one pair) are nearly 

 always in the ninth segment ; in 0. eiseni, however, they are in the eighth 

 segment, and 0. occidentalis is entirely without these organs. 



A fifth point in which the species vary is in the number of the hearts ; in the 

 great majority there are only two pairs of these ; but 0. limicola and 0. hendrici 

 have an additional pair in the ninth segment, which, in the latter species at any rate, 

 is a pulsating vessel like the two which follow. 



Of minor importance, as it appears to me, are the relative size and the lobation 

 of the septal glands, upon which Eisbn is inclined to lay considerable stress ; of less 

 importance still, perhaps, are the relative size and form of the sperm-sacs, where these 

 are — and this applies to the majority — four pairs in segments ix-xii. The degree 

 of sexual maturity would surely be an important factor in causing differences of 

 this kind. The clitellum too shows differences in extent. 



One species, 0. lacuum, differs from the rest in having dorsal pores (at least 

 MlCHAELSF-N has not mentioned them in any species) : in the single pair of testes : 

 in the absence of diverticula to the spermathecae : in the presence of gizzards, and 

 in the saddle-shaped clitellum. On the other hand, it presents the following 

 characteristic features of the genus ; in the male pores being on the seventeenth 

 segment : in the fact that the sperm-duct is enveloped in a muscular sheath : in 



