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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 Somites 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. 
l must confess, however, that Ido 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, ie. 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- 
ties 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 
then 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 
come 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 
indeed that they originated from an outgrowing of the 
