J 008 



front of the end of the notochord and close to the animal 

 pole (Delsman, 1916, 1917). Petromyzon resembles Amphioxus, 

 however, bj the complete segmentation of the mesoderm 

 (Hatschek, 1910) and the separate course of the dorsal and ventral 

 roots of the spinal nerves. The series of well-developed somites con- 

 tinues forward as far as the prostominm. As is the case in most of the 

 Annelids that possess them (Fauvel, 1907, p. IJO), the static vesicles 

 are situated in the second segment of the soma, being the hjoid- or 

 facial-acoustic-segment, but more in the hinder half of it. Thus in 

 Petromyzon and in all Craniotes two pro-otic segments may con- 

 veniently be distinguished, the mandibular or trigeminus-segment 

 and the hyoid- or acustico-facialis-segment. Behind the ear-vesicles 

 then follow the segment of the glossopharyngeus, that of the primary 

 vagus, that of the first spinal nerve, the ganglion of which in 

 Gnathostomes fuses with that of the vagus ("spinalartiger Vagus- 

 anhang", Hatschek, 1892, p. 156) and those of the subsequent spinal 

 nerves. The first pair of somites again send out each a prolongation 

 into the prostomium, the so-called praemandibular somite (Hatschek, 

 1910, p. 481), comparable to the "Kopffortsatz" in Amphioxus, and thus 

 not to be considered as a proper somite. The two pro-otic somites 

 do not contribute any more to the formation of the longitudinal trunk 

 musculature, but together with the so-called "praemandibular somite" 

 give rise to the eye-muscles. The post-otic somites all form regular 

 myotomes, constituting the longitudinal trunk-musculature. This, con- 

 sequently, unlike in other Craniotes, also holds for the glossopha- 

 ryngeus- and the primary-vagus-somite, though in both a beginning 

 of reduction manifests itself in the breaking up of the internal, deeper 

 parts of these myotomes during development (Koltzoff, 1901, p. 329). 

 Evidently this is caused by the strong development of the auditory 

 capsule, which extends backward into the first and the second 

 post-otic segment, the myocommata between the first and the second and 

 between the second and the third myotome as a consequence attaching 

 themselves to the auditory capsule and the first free neural arch being 

 situated between the third and the fourth myotome (cf. e.g. Goodrich, 

 1909, p. 40). Superficially, however, the first and the second myo- 

 tome do not differ from the subsequent ones. 



As in Amphioxus branchiomerism and meso^iierism correspond, 

 the eight gill-pouches, of which the first, the spiracular one, does not 

 break through, alternate with the l^t— 9''' somite (Neal, 1897, p. 447, 

 KoLTZOFF, 1901, p. 432). In front of the first somite lies a median 

 mouth. That the mouth of Craniotes corresponds to two fused gill- 

 slits is a supposition (Dohrn, 1875) which, though not supported 



