74 



ANATOMY OF VEETEBRATES. 



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

 distinctive and peculiar character : in the Brancliiostoma 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 (Adpenser) fig. 62, the growth of cartilage has 

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

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

 cavities for the now well-developed eyes and doul^le nasal sacs : the 

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

 Ijoth sup)ported by a ' \'fimerine ' basis, ff" : beyond which the 

 cranium is continued forward as a long pointed rostrum. The 

 cartilaginous pedicle susj^endlng the palato-maxiilary apparatus is 



Fore part uf ciiaoskelcLuu, Sturgeon 



divided into three piieces ; the epitympanic, ilx m, the mesotvm- 

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

 the j)alatine vault, 20, with wliich the pterygoids, se, are confluent ; 

 the maxillary, 21, the premaxUlary ljone,'2"2, the lalnal cartilage^ 

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

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

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

 the end(3skeleton. The premaxillary is a subtriangular plate, 

 jomed 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 ptervgoid and premaxillary, and 

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

 sym])liysis. 



The mouth of the Sturgeon opens upon the under surface 

 of tlie head, and is protruded and retracted eliieflv by the move- 



