264 
of head segments in such Selachians as Scyllium according 
to ZIEGLER 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. —lf 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 1 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 . 
ZIEGLER were simply branchiomerism, that has nothing to 
do with mesomerism, then again, in studying FRORIEP's 
figures of early stages of development of Elasmobranchs 
(cf. fig. ‚ in which true somites are found also in the 
branchial region, an unprejudiced observer must, 1 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 p.irs 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. 227) 
Thus we must reject FRORI P'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 ar.d 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. 255? The mesomerism 
observed by FRORIEP in nearly the whole chordal part of the 
