THE ANATOMY. SETAE 



' hastate.' The Enchytraeidae show another form of sigmoid seta which in nearly 

 perfectly straight but has not a cleft extremity. 



Another modification of the sigmoid seta is especially characteristic of the family 

 Geoscolicidae among earthworms, though not unknown elsewhere. The end of the 

 seta, which protrudes from the body, is ornamented (woodcut, fig. 2 a. 2, 3) with 

 transverse ridges (Pontoscolex) or with minute spinelets (7Y/V//<7/<a). 



The capilliforin setae, (fig. 2 b. 2) which are exceptionally (Lophochaeta) covered with 



Fig. 20. 



Fig. 26. 



fine processes (woodcut, fig. 2 J>. 5), compar- 

 able perhaps to the ornamentation of the 

 sigmoid setae in Trickochaeta, to some 

 extent graduate into the sigmoid setae. 

 The intermediate condition is seen in 

 Pkreodrilus, where the capilliform setae 

 are shorter, stouter, arid more curved than 

 in other Tubificids. As Phreodrilus is in 

 other respects a link between the Tubifi- 

 cidae and Lumbriculidae, the fact gets 

 an additional interest. 



It sometimes happens that a worm has 

 setae of one kind only from head to tail. 

 This state of affairs however is the less 

 usual. Among the aquatic forms certain 

 species of Aeolosoma, and of the Tubifi- 

 cidae (Limnodrilus), Enchytraeidae, and 

 Lumbriculidae, are the only instances. 

 Among earthworms the Moniligastridae, 

 many Megascolicidae, and some Lumbri- 

 cidae and Eudrilidae are examples. 



The commencement of a diversity 

 in the form of the setae is seen in 

 Phreoryctes, where some of the setae are 



longer than the others the dorsal longer sefcTof Geoscolecid. 3. Trichochaeta lutperidum. 3 b. End of 



, , . , T . . i same more highly magnified. 



than the ventral, or vice versa, or the pos- Fig 2 b , Penial seta of Acanthodri i M gtorg j a n^ ,. Sp<m- 

 terior longer than the anterior ; in many vermafenx. 3 . spiro^erma/erox 4- iivodniut >a*" 



J iMphocTiaeta. ignota. 6. Tubiftx rivulonm. 7. Ifait eltngttit. 



Perichaetidae the ventralmost setae are . xai elinguit. 9. BotumUia amata. 



longer than the others ; in some those upon a few of the anterior segments are longer 



than the rest. 



SETAE OF OLIOOCHAETA. 

 (After Vejdovsky, Stole, Michaelsen.) 



Fig. 20. i. Onychochaeta mndlei : posterior seta. i. Ornamented 

 eta of Geoscolecid. 3. TricJi 

 same more highly magnified. 



