CHAETAE 



351 



this kind ; some of the aquatic Oligochaeta have elongated and 

 hair-like bristles, such as that, of TuUfex. In the Tubificid 

 Lophocliaeta (Fig. 187, 5) the chaetae are ornamented on both 

 sides with delicate processes, which give them the appearance of 

 Crustacean hairs. Among earthworms the simple S-like form 

 is sometimes complicated by the development of sinuous ridges 

 upon the distal end. No doubt these bristles enable their 

 possessor to get a firmer grip of adjacent objects ; they are 

 very commonly found, in the family Geoscolicidae, upon the 

 segments of the clitellum, and permit of a firmer union during 

 sexual congress. In no Oligochaeta are the chaetae borne 

 upon parapodia, as is the case with the Polychaeta ; but in 



FIG. 188. Chaetae. 

 x 10. 1, Ony- 

 chochaeta ; 2, Pon- 

 toscolex ; 3, Tri- 

 chochaeta ; Zb, the 

 same, more highly 

 magnified. 



many of the aquatic forms there are a considerable number 

 to each bundle. In earthworms the number of chaetae 

 varies greatly. The common earthworms of this country, belong- 

 ing to the genera Lumbricus, Allolobophora, and Allurus, have 

 only eight chaetae upon each segment of the body, and these 

 are then, as a rule, arranged in pairs or rather couples, two of 

 each on each side of the body. The genus Perichaeta and some 

 of its allies have a much larger number of chaetae to each 

 segment, disposed in a continuous row round the middle of the 

 segment. The intermediate condition is to be seen in the genus 

 Deinodrilus, where there are twelve in each segment, and in 

 certain members of the genus Megascolex, where there are eight 

 in each segment in the anterior region of the body, the number 

 increasing in the posterior segments. The four bundles of 

 chaetae in the Naids and Tubificids have been likened to the 



