448 



COMPARATIVE ANATOMY. 



contain tlie auditory organ (auditory capsules) (/) are attaclied to the 

 sides of this capsule (Fig. 239, d), and below them two divergent 



bars are given off, which take a 

 carved direction forwards. An- 

 teriorly, these are connected with 

 a process which is given off from 

 the cerebral capsule. An unpaired 

 nasal capsule {<j) is attached to the 

 upper and anterior portion of this 

 latter ; this capsule differs greatly 

 in form in the Myxinoidea and 

 Petromyzontes ; below this there is 

 a broad plate of cartilage, which 

 has a complicated apparatus placed 

 below it, which covers in the oral 

 opening from above (i h I m), and 

 forms a firm framework for the 

 palato-pharyngeal cavity. Poste- 

 riorly the capsule of the skull is 

 continuous with the spinal column. 

 The second form of skull is dis- 

 tinguished by being connected with 

 an apparatus which encloses the 

 opening of the mouth, and which 

 is developed from a branchial arch ; 

 this portion is more or less con- 

 nected with the cranium. Part of it 

 forms the lower jaw, and is always freely movable (Gnathostomata). 

 This arch is differentiated into two pieces, which function as 

 jaws; an upper one, the palato-quadrate, and a lower one, the 



cartilaginous lower jaw. 

 The palato-quadrate is 

 articulated with the base 

 of the skull j but it 

 is enlarged horizontally 

 backwards, and is con- 

 nected with the second 

 arch also, the upper 

 portion of which is 

 movably articulated with 

 the skull. The hyoid 

 forms the lower portion 

 of this second arch. 

 As this upper portion 

 of the second arch is 

 often greatly developed, 

 it gets the appearance of a supporting apparatus for the 

 two primitive jaw parts, which are developed from the first arch, 

 and is known as the hyomandibular. The rudiments of other 



Fig. 239. Skull and coimncnccment 

 of the vertebral colninn of Petro- 

 myzon marinns. A Median sec- 

 tion. B Seen from above, a Note- 

 chord, b Spinal canal, c Rudiments of 

 the vertebral arches, d Cartilaginous 

 cranial cavity, d' Membranous por- 

 tion of the cranial cavity, e Basis 

 cranii. /Auditory capsule, g Nasal 

 capsule. g' Palato-nasal passage. 

 ;/)• Its blind end. 7i Continuation of 

 the bony palate, i Posterior cover, 

 ing-plate of the mouth. Tc Anterior 

 covering-plate. I Labial ring, m Its 

 appendage (after J. Miiller). 



Fig. 240. Skull and visceral skeleton of a Selachian 

 (Diagram), occ Occipital region, la Wall of the 

 labyrinth, eth Ethmoidal region, n Nasal pit. 

 a First, h c Second labial cartilage, o Superior, 

 n Inferior portion of the mandibular arch I. II Hy- 

 oid arch. Ill — VIII (1 — 6) Branchial arches. 



