THE SKULL. 147 



b. The Sense Capsules. The cartilaginous auditory capsules 



appear in tadpoles of about 12 mm. length as thin shells of 

 cartilage investing the auditory vesicles. They are at first 

 quite independent of the cranium, but before the completion of 

 the opercular folds they fuse with the upgrowing parachordals 

 to form part of the side walls of the skull. The pro-otic appears 

 about the time of completion of the metamorphosis. 



The optic capsules are thin shells of cartilage, forming part 

 of the sclerotic coats of the eyes. They arise about the same 

 time as the auditory capsules ; and unlike the other sense 

 capsules, they remain distinct from the cranium throughout life, 

 in order to secure mobility of the eye-balls. 



The olfactory capsules are from their first appearance very 

 closely connected with the anterior ends of the trabeculse, which 

 grow up between them to form the median vertical internasal 

 septum. They develope later than the auditory and optic 

 capsules. 



c. The Visceral Skeleton. This consists of a series of carti- 

 laginous hoops developed within the visceral arches, and forming 

 a framework which surrounds and stiffens the walls of the 

 pharynx. Each hoop consists of right and left halves, which 

 are independent at their dorsal ends, but fused or closely con- 

 nected ventrally. There are in all six of these hoops or bars in 

 the mandibular arch, hyoidean arch, and the four branchial 

 arches respectively ; and they develope in order from before 

 backwards. 



i. The mandibular bar, which is the largest of the series, lies 

 at first parallel to the others, i.e., perpendicular to the long axis 

 of the body. It very early, however, undergoes important 

 changes, and by the time the external gills are developed, and 

 before the appearance of the opercular folds, it has altered its 

 direction, and now runs almost horizontally forwards, parallel 

 to and below the trabeculse. 



It soon imites with the trabeculee, both behind and in front of 

 the eyeball, the latter union being effected by a short transverse 

 bar of cartilage — the palato-pterygoid. In front of the palato- 

 pterygoid, the most anterior part of the mandibular bar becomes 

 segmented off as a short rod of cartilage, which is directed 

 upwards and forwards in the lower lip : it is known as Meckel's 

 cartilage, and forms the basis of the lower jaw or mandible. 



