138 THE ANCESTRY OF VERTEBRATES 



no doubt on its surface, and the olfactory pits, have been 

 lost, together with the cerebral ganglia. Directly under the 

 neuropore lies the anterior end of the notochord and at 

 the left and the right of the latter the first pair of somites 

 (the "mandibular" somites of VAN WYHE, the "coliar- 

 cavities" of MACBRIDE), corresponding to the coelomeso- 

 blast of the first segment, or peristomium, of Annelids. 

 All the somites develop regular and permanent myotomes, 

 from the first onwards. The first pair sends out a prolongation 

 into the prostomium which itself has no primordial meso- 

 derm, while at the same time the fore- end of the notochord 

 secondarily grows out into the snout These "head- or 

 rostral prolongations" of the first pair of somites correspond 

 to the praemandibular cavities of Craniates. Paired gill- 

 pouches are formed each between two somites. The first 

 pair appears in front of the first pair of somites, under 

 the neuropore. The left of them acquires an opening, known 

 as HATSCHEK's pit, to the exterior, the right one gives rise 

 to the praeoral head cavity. Of the second pair, between 

 the first and the second pair of somites, the left one becomes 

 the larval mouih (afterwards the velar opening), the right 

 one the so-called club-shaped glarid which also opens to 

 the exterior but afterwards disappears. Only secondarily, 

 after the "critical stage", does the eumetamerism of gill-slits 

 and somites get lost. The endostyle oriiinates under the 

 mandibular segment, in front of the mouth. 



Spinal gang ia and also head ganglia are not clearly recog- 

 nizable, auditory vesicles are absent. Dorsal and ventral spinal 

 nerves do not unite. There is no distinct difference between 

 the nerves of the anterior segments (corresponding to the head 

 segments of Craniates) and those of the trunk, both being of 

 mixed character. In the region of the former the dorsal roots 

 show a strong plexus-formation and a longitudinal plexus also 

 supplies the gills. It is especially by the strong fourth nerve, 

 evidently comparable to the vagus in Craniates, that this 

 branchial plexus communicates with the medulla and 

 the insignificant brain-vesicle which corresponds to the 

 epichordal deuterencephalon of Craniates. 



The asymmetrical origin cf the mouth no doubt gives us the 

 key to the interpretation of the larval asymmetry, although 

 we shall not indulge now in speculations in this direction. 



Head of Petromyzon. — In Petromyzon part of the surface 

 of the prostomium has been annexed by the medullary tube, 



