35o 



OLIGOCHAETA 



and underneath this again a layer of longitudinal muscles. In 

 both layers the fibres have a softer core, outside which lies the 

 radially striated muscular substance. The fibres are embedded 

 in a granular matrix. It used to be considered at one time 

 that such medullated fibres were distinctive of leeches as opposed 

 to Oligochaeta. Their existence has been really known in the 

 Oligochaeta since the researches of Eatzel ; but Cerfontaine has 

 fully described them, and emphasised the fact that the fibres of 

 both circular and longitudinal coats are alike in this respect. 



Chaetae. The passive organs of locomotion in these animals 

 are the chaetae, which are absent in only one family, Discodrilidae, 

 and in one other genus, Anachaeta. In this latter worm the 

 chaetae are represented by large glandular cells, which seem to 



Fig. 187. Chaetae of 

 Oligochaeta. x 10. 

 (After Michaelsen,* 

 Stole, and Vejdovsky ). 

 1, 2, Penial chaetae of 

 A cantlwdrilus geor- 

 gianus; 3, Spiro- 



s per ma 

 drilus , 

 chaeta ; 6, 

 7, 8, Nais ; 

 milla. Figs, 

 ordinary chaetae. 



4, Ilyo- 



5, Lopho- 



, Tubifex; 



9, Bohe- 



3-9 are 



correspond to the cells from which the chaetae arise in other 

 forms. They are in this case,, as in the others, cells of the 

 epidermis. The chaetae of the Oligochaeta are not quite so 

 variable in form as in the marine Polychaeta (see Fig. 138, p. 

 267). Figs. 187 and 188 illustrate some of the principal 

 shapes which these bristles assume. The most prevalent form 

 is an elongated S, which has been aptly compared to the 

 mathematical sign J. This kind of chaeta is found in all 

 earthworms, and in not a few aquatic genera such as the Lum- 

 briculidae. In some of the latter and in the Tubificidae and 

 Xaids there is the same form of chaeta, which is cleft at the 

 free end, and possibly enables the worm to grasp the leaves of 

 aquatic plants, and otherwise facilitates progression in a laxer 

 medium than the stiff soil frequented by the earthworms. Even 

 earthworms, at any rate the genus Pontoscolex, have chaetae of 



