392 OLIGOCHAETA 



out of which it will be formed, are provided with setae just as are the other segments 

 of the body ; every intermediate condition almost occurs ; thus there are species 

 with a few setae on the ventral surface of the clitellar segments, and species in 

 which the setae are limited to the last segment of the clitellum, &c. It is a rule, 

 subject to only one exception, that the setae upon the clitellum are in no way 

 different from those found elsewhere ; the exception is P. houlleti ; in this worm 

 the clitellar setae are very short, and are bifid at the free extremity. 



(4) It is a very general rule among the members of the genus Perichaeta that 

 the clitellum is limited to the three segments xiv, xv, xvi. Whether it is ever less 

 than that seems to me to be doubtful ; it is true that several species, such as P. bicincta, 

 have been described in which the clitellum is still further limited, and consists of the 

 two segments xiv and xv ; I have found this to occur in P. upoluensis. I should 

 wish, however, to examine a large series of specimens of this and other species, in 

 which the character has been described, before considering it to be established. As to 

 species in which the clitellum occupies a greater number of segments than three, there is 

 but one, unless some of the species referred to here on p. 369 as incertae sedis are really 

 true Perichaetae. Another point of importance to be considered in describing species 

 of Perichaeta is the exact way in which the clitellum terminates and commences ; in 

 some species it fully occupies the three segments xiv-xvi ; in others it does not begin 

 before about the middle of segment xiv, and it may also terminate before the end of 

 segment xvi. In the description of the species which follows I have taken pains 

 to indicate this peculiarity in the clitellum by placing a line above or below the 

 number of the segment at which it Commences ; a line above indicates that the 

 clitellum does not commence sharp with the commencement of the segment ; a line 

 below, that it does not end sharply with its termination. The clitellum is invariably 

 complete, and is developed ventrally as well as dorsally. 



(5) BENHAM (2) has grouped the members of the genus according as to whether 

 they do or do not possess copulatory papillae ; BOURNE (4) has objected to this 

 classification. No doubt it is difficult to prove a negative in the case of some 

 species, and besides one character alone is dangerous to adopt. It will be noticed, 

 however, in the table of the characters given on pp. 399-401 that the absence of genital 

 papillae goes with the presence of a terminal muscular sac to the atrial duct. It 

 is true that the converse does not invariably hold, but there is no case of a Perichaeta 

 with a terminal sac to the atrium which at the same time possesses genital papillae. 

 It is quite likely that there is some physiological relation between these structures ; 

 the absence of this eversible sac at the extremity of the atrial duct may render the 

 papillae necessary for the two individuals to adhere to each other during coitus. 



