48 THE ANCESTRY OF VERTEBRATES 



behind the auditory capsule only, the latter being considered 

 by HatscHEK as belonging to the unsegmented anterior end- 

 of the Annelid, distinguished by him as the head (cf. anon). 

 Kleinenberg (1886, p. 190) has shown the inadequacy of 

 HATSCHEK's definition of the head and has replaced it by 

 another by which this term was restricted to the prostomium* 

 and the statocysts were counted to the segmented soma. 

 Yet the suggestion made here by HATSCHEK agrees closely 

 with the second fundamental idea of my theory which at 

 first sight might appear so hazardous thas i was glad tO' 

 find a predecessor like HATSCHEK. 



Brain of Annelids and Craniates. — A question which now 

 presents itself is, whether it might be possible to trace back 

 the foundations upon which in the course of phylogeny 

 the complicated brain of higher Craniates has built itself 

 up to certain structures of the apical plate of Annelids. The 

 cerebral ganglion or brain of Annelids does not represent 

 a simple structure but is composed of several centres, 

 each having a separate function and origin. According to 

 HATSCHEK (1891, p. 424) and Racovitza (1896) we may 

 distinguish in the prostomium from in front backward : 



1. an anterior pair of "Tentacularganglien" or "cerveau 

 anterieur," innervating the so-called primary tentacles or 

 palps which originate at both sides of the larval apical 

 sense organ at the animal pole, 



2. a "Mittelhiin" or "cerveau moyen," innervating the 

 eyes and the antennae, 



3. a pair of "Riechlappen" or "cerveau posterieur" inner- 

 vating the ciliated pits 



Each of these three centres owes its origin phylogenetically 

 and ontogenetically to a sense-organ of the praeoral lobe; the 

 nervous centre is produced by the sense-organ innervated 

 by it, as stated by KLEINENBERG (1886). Three sensitivo- 

 nervous regions then may be distinguished in the cephalic 

 lobe: "la region palpaire est gustative et tactile, la region 

 sincipitale est tactile et visuelle, la region nucale est olfactive". 

 Often the ganglia in Annelids remain in direct contact and 

 are connected to the part of the epidermis from which they 

 have originated and with the sense-organs to which they 

 belong. Often, however, part of the ganglion detaches itself and 

 now lies in the interior of the praeoral lobe, connected by 

 a short nerve with the other part which remains applied 

 to the sense-organ. In this way three centres of the Annelid 



