EVOLUTION OP ORGANS IN THE CHORDATA. 277 



The musculature of the hyoid arch is peculiar in this respect, 

 that no internal portion of the muscle-tube is segmented off 

 by the cartilage, and accordingly no adductor is formed. The 

 musculi interarcuales are also absent, and there is a complicated 

 system of ligaments fastening the hyomandibular cartilage. 

 The ventral muscles, on the other hand, are similar to those of 

 the posterior arches. 



With regard to the cartilage of the hyoid arch, development 

 shows that in the Sharks the upper middle internode, dorsal to 

 the venous commissure forms the hyomandibular, no separate 

 basale or dorsal internode is formed; but, as the hyomandi- 

 bular carries a number of branchial cartilage rays, and also a 

 dorsal ray, which is homodynamous with the upper extra- 

 branchial (so-called) cartilage of the gill arches, it follows that 

 the hyomandibular contains the basale (dorsal internode) of 

 the hyoid arch. 



In the Sharks the first rudiment of the mandibular arch 

 appears at the level where the hyoid vein joins the spiracular 

 artery, but unlike the posterior rudiments it consists from 

 the first of two cartilaginous centres : the under becomes the 

 mandible, the upper the upper jaw, the so-called palato-quad- 

 rate. No adductor is formed in the mandibular arch. It has 

 been generally taught that the masticatory muscle of the jaws 

 is the homologue of the adductor, but this is not so ; no homo- 

 logue of the adductor is present. 



There are no cartilaginous rays on the mandibular arch. 

 The doctrine, therefore, of Gegenbauer and his followers, that 

 the lower and upper jaw are parts of a single cartilage arch 

 equivalent to a posterior gill arch is unfounded. 



In the Rays the development of the cartilages of the hyoid 

 arch is quite different to that described for the Sharks. 



There are two cartilage-centres, one near the posterior edge 

 of the arch, the other near the anterior side, behind the spira- 

 cular cleft ; each cartilage has its own muscle system. The 

 first cartilage is separated into a dorsal and ventral part by the 

 venous commissure, and each part bears gill rays. The second 

 cartilage becomes the hyomandibular, it has its own muscle 



