74 



ANATOMY OF VERTEBRATES. 



proceeds in a special direction, to stamp the species with its own 

 distinctive and peculiar character : in the Branchiostoma by the 

 articulated cartilaginous labial arch and its numerous filaments ; 

 and in the proper Myxinoids and Lampreys by the formation of 

 the complex system of lateral and labial cartilages ; or by the 

 modification of the palatine, maxillary, and hyoid rudiments, in 

 relation to the suctorial function of the mouth. 



In the Sturgeon (Acipenser) fig. 62, the growth of cartilage has 

 inclosed the whole of the brain-case, f, y, and blended with its 

 walls the ear-capsules : in advance of this it developes protective 

 cavities for the now well-developed eyes and double nasal sacs : the 

 orbit, i, being divided from the nostril, k, by the ridge, 2, and 

 both supported by a ' vomerine ' basis, cf" : beyond which the 

 cranium is continued forward as a long pointed rostrum. The 

 cartilaginous pedicle suspending the palato-maxillary apparatus is 



Fore part of endoskeleton, Sturgeon 



divided into three pieces ; the epitympanic, ib. m, the mesotym- 

 panic, ib. n, and the hypotympanic, ib. 26. The latter supports 

 the palatine vault, 20, with which the pterygoids, se, are confluent ; 

 the maxillary, 21, the premaxillary bone, 22, the labial cartilage, 

 74, and the mandible, 32. All these parts of the edentulous 

 suctorial mouth are very small in proportion to the size of the 

 head and entire fish ; and they are the only ossified parts of 

 the endoskeleton. The premaxillary is a subtriaugular plate, 

 joined by ligament to its fellow, trenchant anteriorly, and 

 extending in an arched form to the mandible. The mandible, 32, 

 articulates by a concavity to the pterygoid and premaxillary, and 

 consists of a single piece, united to its fellow by a ligamentous 

 symphysis. 



The mouth of the Sturgeon opens upon the under surface 

 of the head, and is protruded and retracted chiefly by the move- 



