Scharff — On Dohrn's Theories on the Origin of Vertebrates. 27 

 and each unites with a vessel coming from the spiracle. (See fig. 



VIII.) 



One of the most characteristic differences between the muscular 

 system of the hyoid arch and that of the branchial arches lies in 

 the absence of the proximal piece from which, in the latter, the 

 adductor takes its origin. The musculi interarcuales are likewise 

 wanting ; but instead of these muscles we find a complicated arrange- 

 ment of ligaments, fixing the so-called hyomandibular. The ventral 

 muscles, however, are present, and are like those of the posterior 

 arches. 



The venous commissure in the hyoid arch indicates the position 

 where the primitive cartilage is formed — in fact it lies in the 



th. a. 



Anterior portion of arterial circulatory system in an embryo of Pristiurus (after Dohrn). 



th 



sp. a 



h.a 



h.v 



hy 



= thyroid. 



= spiracular artery. 



= hyoid artery. 



= hyoid vein. 



= hyoid arch. 



p.v 

 v. c 

 a. v 

 con 

 th.a 



= post. vein. 



= venous commissure. 



= ant. vein. 



= conus arteriosus. 



= thyroid artery. 



middle of the hyoid cartilage. In the sharks we recognize the 

 hyomandibular as the upper middle piece of the hyoid arch by its 

 supporting a number of cartilaginous rays. A dorsal ray homo- 

 dynamous with the extra-branchial rays is also present. Hence 

 it follows that the hyomandibular likewise contains the " basale " 

 (pharyngobranchial) of the hyoid arch. 



The first rudiment of the mandibular arch in sharks appears on 

 the same level where the rudimentary hyoid cartilage is formed. 

 The difference, however, in this formation is made apparent by the 

 circumstance that the mandibular cartilages arise from two centres, 

 one of which becomes the lower jaw, and the other the upper jaw, 

 the so-called palato-quadrate. A segmentation of the cartilage 

 into "middle piece," "basale," and "copulare" does not take 

 place. No venous commissure is formed, and no adductor. As the 



