1901.] OBTAIIfED DTTRTNG THE " SKEAT EXPEDITIOIS"." 83 



to be seen at the periphery o£ the organ, the central part corre- 

 sponding to the external depression consisting of rather less 

 elongated cells. Tlie considerable size of the organ as compared 

 with the entire prostomiiim is apparent from the fignre and is 

 rather remarkable. It is probable, therefore, that a corresponding 

 organ in other Tubificidse has not been overlooked. 



The seUe are, as in the other species, all uncinate and without 

 further complications : there are no subsidiary booklets between 

 the two prongs in which the free extremity of the seta ends. 

 There appear to be not more than four set?e to a bundle, and very 

 often there are only two. 



In tliis species, as in other Oligochaeta, there would appear to 

 be no setse upon the first segment of the body. However, in 

 longitudinal sections I was able to observe a small mass of muscles 

 upon the first segment of the body, entirely similar in appearance 

 to those which upon ensuing segments enwrap the setfe, and 

 corresponding exactly in position. The mass of muscular fibres 

 was small, but I regard it as a vestige of the seta-bundles of that 

 segment. Setae are certainly absent ventrally in the neighbourhood 

 of the male pore — a feature in which the px'esent species agrees with 

 B. americaymm, and differs from the European B. vejdovshyanum, 

 in which there are specially modified genital setse of peculiar form 

 replacing the ventral bundles. The lateral setae, however, corre- 

 sponding to the missing ventral setse are present. 



CUteUum. — Bothrioneuron iris differs from its allies in the position 

 of the clitellum, which is pushed back a segment and occupies 

 segments xii. and xiii. In the middle of segment xii. lies the 



Alale generative pore. — This aperture, single and median, is also 

 a segment farther back than it is in B. vejdovskyanum and 

 B. americanum. It is interesting to note that there is an apparent 

 connection between the male pore and the clitellum in that an 

 alteration in the position of one is accompanied by an alteration in 

 the position of the other. 



The oviducal pores I have not seen,. 



Spermatliecal pores are not present. The absence of spermathecse 

 is one of the characters of the genus. 



Spermatopliores. — -Dr. Stole figures in B. vejdovshyanum a crowd 

 of sperraatophores attached to the body-wall round the male 

 generative pore. In B. americanum I did not find these structures, 

 though perhaps I was a little premature ^ in using their absence as 

 a specific character. In three out of six mature examples of B. iris, 

 I find a single spermatophore apiece also attached close to the 

 male pore. The structure of these sperraatophores is similar to 

 those of B. vejdovshyanum. There is a thick stalk by which they 

 are attached to the body-wall, which is of a yellowish colour. This 

 stalk is merely attached to the epidermis superficially : it does not 

 penetrate between the cells. Nor can I find any evidence of its 



1 Monogr. Oligocha'ta, Oxford, 1895. The comparative rarity'of the occur- 

 rence of spermatophores in B. iris may explain the failure to find them in 

 B. americanum. Tliey are probably distinctive of the genus. 



6* 



