108 



THE ANATOMY OF VERTEBRATED ANIMALS. 



oxus ^ and the PacJiycardia^ comprising all other Vercejrata. 

 The great peculiarities in the development of Amphioxus, 

 and the many analogies with invertebrate animals, particu- 

 larly the Ascidians, which it presents, lend much support to 

 this proposition. 



No fossil form allied to A)n2^hioxics is known. 



II. The Maesipobranciiii. — In this order of the class 

 Pisces the integument is devoid of scales or bony plates. 



The spinal column consists of a thick persistent notochord 

 enveloped in a sheath, but devoid of vertebral centra. The 

 neural arches and the ribs may be represented by cartilages, 

 and there is a distinct si^ull presenting cartilage at least in its 

 base, and retaining many of the characters of the foetal cra- 

 nium of the higher Vertebrata. The notochord terminates in 

 a point in the base of this cartilaginous skull behind the pitui- 

 tary body ; and the skull is not movable upon the spinal col- 



fio. 80. — A, the skull of a Lamprey, viewed from the side ; B, from above : — «, the ethmovo- 

 merine plate ; h, the olfactory oapsiilo ; c, the auditory capsule ; </. the neural arches of 

 the spinal column ; e, the i)alatopterypoid portion ; f, probably, the metapterypoid, or 

 Buporior quadrate, portion, and g, the inferior quadrate portion, of the sul>ocular arch ; /;, 

 otylohyal process; i. Ungual cartilage; ^•, inferior, lateral, prolongation of the cratii.sl 

 cartilage; 1, 2, 3, accessory labial cartilages; m, branchial skeleton. Thp. spaces ou 

 eltJier side of 1 are closed bv membrane. 



