170 



CHORDATE ANATOMY 



bones of lower vertebrates develop in the otic capsule and help to form 

 the petrosal and mastoid portions of the temporal bone. The styloid 

 process of the temporal is an ossified portion of the hyoid cartilage which 

 fuses with the temporal. The squamous portion of the temporal is mem- 

 branous in origin. An outgrowth of the epitheUum of the middle ear 

 penetrates the mastoid process to form a cavity or antrum. 



The rest of the cranium is membrane bone. Because the roofing 

 bones of the brain case ossify slowly and expand from centers, uncovered 

 regions or fontanelles persist for some months after birth as "soft-spots" 

 between the frontal, parietal, and occipital bones. 



lo Idm ^° ^9 <fc bh /^ 



Fig. i6o. — Ventral and lateral views of the skull of lamprey (Petromyzon marinus). 

 ad, anterior dorsal cartilage; bb, branchial basket; gc, gill cleft; Ic, labial cartilage; 

 Idm, lateral distal mandibular; Ig, lingual cartilage; nc, nasal capsule; oc, otic capsule; on, 

 optic nerve; pc, pericardial cartilage; pd, posterior dorsal cartilage. (After Parker from 

 Kingsley's "Comparative Anatomy of Vertebrates.") 



Evolution of the Visceral Skeleton. Evidence from both comparative 

 anatomy and embryology indicates that the upper and lower jaws, the 

 hyoid bone, the ear bones, and the laryngeal cartilages of man have evolved 

 from the skeletal gill supports of primitive fishes. 



Cartilaginous supports for the respiratory system are lacking in the 

 lowest chordates. In Amphioxus, the velum in which the mouth lies 

 and the gill arches are supported by slender rods of a material which, from 

 its resemblance to cartilage, is called pro-cartilage. A truly cartilaginous 

 visceral skeleton first appears in cyclostomes in the form of a gill-basket in 

 which the number of cartilage rods corresponds with the number of gill- 

 arches. Bdellostoma may have as many as fifteen of these, other cyclo- 

 stomes eight or nine. 



