ORIGIN AND STRUCTURE OF THE HEAD 85 



communication with the pericard. This, truly, need not 

 "surprise us, if we imagine with DOHRN that the mouth 

 originated by the union of two gill-slits; in this way the 

 communication of the praeoral s^mttes with the pericardial 

 cavity was of course cut off. Owing to this, and by their minute 

 size, the praemandibular somites are distinguished from 

 those following. When first formed in Selachians, they remain 

 connected with each other for some time by a little stalk 

 in front of the. notochord. Soon afterwards this connection 

 is lost. Thus the secondary mouth of Vertebrates would 

 have pierced between the first and the second segment of 

 the soma, while the primary mouth, that of Annelids, the 

 neuropore of Amphioxus, is to be found just in front of 

 the first segment. 



I must confess, however, that I do not feel convinced that 

 the praemandibular cavities represent the first segment and 

 that they are not to be considered as a detached part of 

 the mandibular segment, which has applied itself to the eye 

 vesicle, i.e. a prostomial organ. The circumstance espe- 

 'Cially, that the eyes are organs belonging to the prostomium, 

 made me ask the questions: do not the praemandibular cavi- 

 iies lie in the prostomium, does not the mouth indicate ventrally 

 the limit of the prostomium and the first segment? If the 

 branchial diverticula of the gut made their way each in the 

 groove between two segments, perhaps finding here places 

 of minor resistance, or simply as a consequence of the 

 regular alternation of gut diverticula and genital follicles 

 present already in Plathelminthes, might it not be expected 

 ihen in some degree, that the first opening to the exterior 

 would break through in front of the first segment, which 

 •in this case would be the mandibular segment? Then in 

 Selachians like Scyllium and Pristiurus the number of 

 'Visceral archs would prove to correspond fully to the 

 number of segments incorporated into the head. We should 

 icome to the conclusion that the new mouth of Vertebrates 

 has broken through exactly opposite the old mouth of 

 Annelids, like the latter between the prostomium and the 

 first segment, and — though this of course cannot serve as an 

 argument — the name prostomium ("in front of the mouth") 

 might be used with equal right in Vertebrates as in Annelids. 



Balfour (1878, p. 206), who first described the praeman- 

 dibular cavities, and his disciple MARSHALL (1879) thought 

 nndeed that they originated from an outgrowing of the 



