THE ANATOMY. CEPH ALIS ATION 11 



the primitive position of the Perichaetidae among earthworms can also be used ; 

 this matter however is discussed later. SPENCER has referred to the existence 

 in Metjascolex of what would seem to be a regenerating tail ; and it is noteworthy 

 that here the setae are more numerous than anteriorly ; it suggests a recurrence to 

 an earlier condition. Why the number should have been reduced to eight exactly 

 is a more difficult matter ; it is a problem comparable to that involved in the attempted 

 explanation of why earthworms should usually have two pairs of teates and certain 

 species four pairs of spermathecae. There is no obvious advantage to be discerned 

 in either fact. 



Typically the setae are repeated from segment to segment, with or without 

 modification in form in different parts of the body or in different regions of the same 

 segment. It sometimes happens, however, that the setae are partially or entirely missing 

 upon certain segments of the body. These segments are either the first few segments 

 of the body or certain of the genital segments. I have already mentioned the entire 

 absence of dorsal setae in Chaetogiitter and of all the setae in Anuchaeta. 



In the segments which bear the male pores the ventral setae are very commonly 

 absent ; this is the case for example with Dero and with a good many earthworms 

 (e.g. Ocnerodrilus, Megascolides orthostichon) ; in Psammoryctex l> r/itn the dorsal 

 setae also of the segment (eleventh) which bears the male pores are absent. 



A more remarkable instance of a specialisation of this kind occurs in a large 

 number of species of Perichaeta. In those species, for example in the common 

 P. indica, the three segments of the clitellum are quite without setae. Various inter- 

 mediate conditions between the total absence of setae and their presence to the full 

 number are seen in other species. There seems also to be a tendency in other 

 earthworms for the clitellar setae to disappear, though sometimes, as has been already 

 mentioned, the converse occurs, and they become replaced by a series of a different 

 form. 



5. Cephalisation. LANKESTER has applied this expression to the specialisation of 

 the anterior segments of the body so frequently seen among the Oligochaeta. As already 

 mentioned, all Oligochaeta show cephalisation as regards the first segment of the body, 

 which never possesses setae. There are a few earthworms in which more than the first 

 segments of the body are without setae ; these worms chiefly belong to the family 

 Geoscolicidae, and the number of segments which are thus without setae differ in 

 different species. There are as many as twenty in Kyiiofti*. The Geoscolicidae are 

 not the only family which show this character; in the genus l)i'ili-itt(*. belonging 

 to the Cryptodrilidae, the first segments are similarly devoid of setae. Among the 

 Naidomorpha, the cephalisation affects the dorsal bundles of setae only : in Chaeto- 



c 2 



