78 THE ANCESTRY OF VERTEBRATES 



of head segments in such Selachians as Scyllium according 

 to ZlEGLER is one (the praemandibular segment) more 

 than the number of visceral archs, being accordingly eight. 

 According to VAN Wyhe, who considers the hyoid segment 

 as a double segment, it amounts to nine, the same number 

 as reached originally by Gegenbaur, though their segments 

 do not fully coincide. 



Auditory vesicle in second segment — If now in this scheme 

 we look at the situation of the auditory vesicle, it is evident 

 that there can be no doubt, that it belongs to the segmented 

 region of the body and lies over the third or hyoid seg- 

 ment, if we count the praemandibular cavities as the first 

 pair of segments. Now, as I will explain below, 1 do not 

 feel convinced that the praemandibular cavities indeed re- 

 present a first pair of mesoderm segments, and I will give 

 some reasons, which seem to me to plead for the view 

 that the mandibular segment is to be considered as the 

 first. If this view be right, we come to the conclusion 

 that the auditory vesicle belongs to the second body segment. 



On the other hand, if, with FRORIEP, we assume that 

 the segmentation of the head observed by VAN WYHE and 

 ZlEGLER were simply branchiomerism, that has nothing to 

 do with mesomerism, then again, in studying FRORlEP's 

 figures of early stages of development of Elasmobranchs 

 (cf. fig 16), in which true somites are found also in the 

 branchial region, an unprejudiced observer must, I think, 

 come to the conclusion, that the auditory vesicle (placode) 

 lies over the second segment. 



in Annelids also, where they occur, the statocysts belong 

 to one of the first somatic segments, not to the prostomium 

 or to the head, as first suggested by HATSCHEK (cf. fig. 14). 

 As we have seen, there are often several pairs in more 

 primitive forms, while in more differentiated forms they are 

 restricted to one segment only, which in the majority of 

 cases is equally the second segment (cf. p. 41) 



Thus we must reject FRORlEP's view that the auditory 

 organs in Vertebrates, just as the optic and olfactory organs, 

 belong to the primarily unsegmented anterior part of the head. 



Branchiomerism and mesomerism. — We now come to the 

 third question: does the branchiomerism correspond to the 

 mesomerism and do the gill-slits belong to the first or to the 

 second region mentioned sub 2 on p. 69? The mesomerism 

 observed by FRORIEP in nearly the whole chordal part of the 



