192 



OX THE STRUCTURE OF THE SKULL. 



tion of its characters in the former will suffice to illustrate its 

 nature. Fig. 75 represents a vertical and longitudinal section 



Fig. 75. 



Fig. 75. — Vertical and longitudinal section of the anterior part of the body of a Lamprey 

 (Petrom;/zon marinus). — A, the cranium, with its contained brain ; a, section of the 

 edge of the cartilage marked a in Fig. 76 ; Olf, the entrance to the olfactory chamber, 

 which is prolonged into the caeca 1 pouch, o ; Ph, the pharynx ; Br, the branchial 

 channel, with the inner apertures ot the branchial sacs ; 31, the cavity of the mouth, 

 with its homy teeth ; 2, the cartilage which supports the tongue ; 3, the oral ring. 



of the anterior part of the body of the large Sea Lamprey (Petro- 

 myzon marinus), and gives a very good notion of the excessively 

 minute proportions of the proper skull (A) to the rest of the 

 body in this animal. A and B (Fig. 76) are lateral and superior 

 views of the skull with its accessory cartilages, separated from 

 the soft parts. The notochord (Gli) is, as in AmjjJiioxus, ex- 

 ceedingly large, and is surrounded by a merely membranous 

 sheath, from which prolongations are given off above to form the 

 sides of the small neural canal. In the walls of this canal, carti- 

 laginous rods, which represent neural arches, are developed, and 

 it dilates more distinctly in the head than in Ampliioxus, 

 though the cranial cavity is still very minute. The myelon 

 also undergoes a very distinct enlargement as it enters the 

 cavity, and all the typical divisions of the vertebrate ence- 

 phalon are recognisable in the brain thus formed. 



The notochord terminates in a point immediately behind 

 the pituitary body. As it approaches the cranium, a rod of 

 cartilage (I, Fig. 76, A) is developed on each side in the lateral 

 parts of its sheath, and gives attachment to the branchial 

 skeleton (in) ; still more anteriorly two other cartilaginous fila- 



