1180 



SCANDINAVIAN FISHES. 



alt 

 iiiU 



^^^K^S^l - -am 





""'^'' :/*jiji 



Ml (I, I lap end 



Fig. 350. Section of the forepart of a Lampern (Petromi/zon); about 

 twice (lie nat. size; partly after Parker, partly after Vogt and Yung. 

 all); branchial artery; nhy, anterior liyoid cartilage; aln, anterior 

 linguodental plate; alt, anterior lateral dental plate; (iiui, annular car- 



tilage of the mouth (severed in two places); at, anterior dental plate; 

 aiirc, auricle of the heart ; c, ventricle ; Uer I + II, prosencephalon 

 and thalamencephalon ; Uer III, mesencephalon; Cer IV + V, cerebellum 

 and medulla oblongata; ch, chorda dorsalis; coel, abdominal cavity 

 (coelom) ; ctr, cornua trabecularum ; dc, ductus Cuvieri ; ebr, outer gill- 

 openings; hej}, liver; Hy, nostril; ibr I^VII, first-seventh inner gill- 

 openings; med, median upper cartilage of the mouth; Ml, myelon; 

 mtt, left mediolateral dental plate; mr/c, mj-ocomma; mt/m, mj-omere; 

 11, olfactory capsule (the severed cartilaginous wall); oes, oesophagus; 

 ovar, anterior extremity of ovary; pliy, posterior liyoid cartilage; pt, 

 posterior dental plate of the mouth: t. tegmen oranii : r. anterior 

 part of intestine; vel, vehim. 



paired eyes is connected, and its manner of origin is 

 the same, the difference is strictly but local. In its 

 highest development too, it becomes an eye with optic 

 nerve, an eye in the middle of the skull, asymmetrical, 

 for its nerve root would apjiear to originate from the 

 left optic thalamus, though l)efore this becomes a true 

 optic thalamus, and wliile it still lies, as in the Lanii)re\'s, 

 in the roof of the bi'ain and bears the name oi' fffDif/Jion 

 liaheniiJfP {intermedium) siiiisfniiiL This pineal eye, 

 ■whose functional time jjroper fell under the period known 

 by geologists as the Mesozoic, bore within itself, how- 

 ever, the seeds of its destruction in the vertebrates, for 

 it was constructed on the ocular type of the invertebrates: 

 — its lens was an ependymal instead of an epithelial 

 (epiblastic) growth, and its retinal cells had the base 

 directed peripherally instead of centripetally into the 

 eyeball. And after the advent of the Tertiar^■ ]>eriod this 

 Polyphemous type disappeared from among the verteb- 

 rates. In many, however, as in the Lampreys, it endures 

 in a more or less vestigial condition. To all ap])earances 

 the pineal eye was more ancient than the ])aired eyes, 

 perhaps originally the only true eye in the worm-like 

 ancestors of the vertebrates, hardly elevated as yet above 

 the level of the invertebrates; and all the sensations des- 

 tined for our consciousness still follow in tlie optic thalami 

 (if we maj' believe LuYS, fig. 348) a path laid down 

 through the realm where the pineal eye once held sway. 

 The mental faculties of the higher vertebrates 

 had a long history before thev became what they 

 are. The pineal eve and the sensory organs of the 

 primitive gill-clefts are examples of structures at dif- 

 ferent grades of abortion. The first has almost ceased 

 to functionate at all, and terminates its existence as 

 a functional organ in the class of reptiles; the latter 

 do not extend so high in the animal series, and they 

 nowhere retain their original, preponderating signifi- 

 cance; but they have a descendant, the system of the 



