56 THE ANCESTRY OF VERTEBRATES 



neural longitudinal musculature, i. e. the lateral line, in which 

 they are implanted. This line does not always lie exactly 

 halfway between the two parapodia of one side but, e. g., 

 at the beginning of the abdomen rises more to the dorsal 

 side and at the end sinks more to the neural side, thus 

 forming a slightly curved S. These knobs are provided 

 in the centre with extremely fine sense-hairs. In this respect 

 these sense-organs show an undeniable resemblance to those 

 of Vertebrates: in both the central cells terminate in fine 

 stiff cilia and are surrounded by so-called supporting cells. 

 The number of sense-hairs is subject to variation in diffe- 

 rent Vertebrates which possess lateral sense-organs, but is 

 always considerably less than in Capitellids. Their length 

 also appears to be less. Also in other families of polychae- 

 tous and oligochaetous Annelids similar sense-organs have 

 been described, while moreover the dorsal cirri of the ven- 

 tral parapodia are considered homologous to them by ElSlG 

 (1887, p. 512), who in this respect follows Kleinenberg. 



Metameric arrangement. — Great weight is laid by ElSlG 

 on the metameric arrangement of the lateral sense-organs. 

 In Vertebrates this is clearly pronounced in the ontogeny 

 of Telosteans, and in other groups it is rendered probable 

 by several observations. In part, the occurence of more 

 than one sense-organ in one segment, is accounted for by 

 the faculty of dividing which was first demonstrated by 

 MALBRANC (1876) and afterwards confirmed by others. But 

 only in Teleosteans a truly metameric arrangement of the 

 sense-organs at their first appearance has been observed, 

 which, however, is considered by some investigators as a 

 secondarily acquired character. It is not impossible, that a 

 primitive metameric arrangement of the lateral organs might 

 have been gradually obscured when the external segment- 

 ation, so strongly pronounced in the chitin-coated Annelids, 

 became less distinct in Vertebrates, and only in such cases 

 might have been preserved, or have reappeared, where 

 the integument shows metameric segmentation, as e.g. in 

 Siphonops and in Ichthyophis. and in Teleosteans, where the 

 arrangement of the scales in general corresponds to that 

 of the myotoms 



Innervation. — If now we consider the innervation of the 

 lateral sense-organs in Vertebrates and Annelids, it appears 

 that this does not correspond in the two groups. In Capi- 

 tellids the lateral organs from segment to segment are 



