DESCRIPTIONS OF GENERA AND SPECIES 359 



modified clitellar setae ; to this rule there is but one exception, viz. Pericliaeta houlleti ; 

 and in this worm the modified genital setae are not at all like those of the Lumbricidae 

 or the Geoscolicidae. 



The mutual relationships of the three subfamilies are more fully discussed in the 

 following pages. 



FAMILY PERICHAETIDAE. 



DEFINITION. Earthworms with numerous setae per segment arranged in a con- 

 tinuous circle, or with dorsal and ventral gaps ; male pore nearly always on 

 XVIII. Gizzard always present ; calciferous glands absent or present ; intestine 

 frequently with, two (occasionally more) caeca, and a rudimentary typhlosole. 

 Nephridia diffused or paired. Spermiducal glands generally lobate ; penial setae 

 present or absent ; spermathecae with one or two diverticula '. 



This family of earthworms is one of the largest; it comprises over 100 species, 

 which will be in this work relegated to four genera. This family, like many others, 

 contains some of the largest as well as some of the smallest earthworms. There are 

 no species known which are of greater size than Perichaeta musica, while other 

 species do not reach to a greater length than two inches. 



Anatomical Characters 2 . 



The continuous circle of setae does not absolutely distinguish the family as it was 

 formerly thought to do. I am quite of BENHAM'S opinion that his genus Plagiochaeta 

 is an Acanthodrilid. The setae are in the species of the genus Megascolex interrupted 

 dorsall) r and also ventrally by a gap where setae are absent. In the remarkable 

 species which I formerly placed in the genus Anisochaeta there is an interesting 

 transition to the worms with eight setae to the segment ; that is what is found in 

 the anterior segments of those species, while posteriorly the numerous setae of the 

 typical species exist. As a general rule the setae are less numerous in the most 

 anterior segments the reason for which may be no more recondite than that these 

 segments are of less circumference ; the number of setae to the segment varies and 

 often affords a valuable series of characters for the systematist; the largest number 

 of setae in a single segment is shown in Pleionogaster horsti, where there may be 

 as many as 151. 



1 Very exceptionally absent P. ijima. 



3 For the affinities of the Perifchaetidao see under Cryptodrilidae and Acantliodrilidae. 



