LANCELOT. GEOLOGICAL DISTRIBUTION OF FISHES. G5 



rulists, been removed altogether from the Vertebrated sub-king- 

 dom, on account of the almost complete absence of what are 

 usually regarded as the distinguishing peculiarities of that group. 

 Nevertheless, an attentive examination of its structure shows, 

 that it bears a closer resemblance to the true Fishes than to 

 any other animals exhibiting their conformation in (as it were) 

 a degraded form. It is of very diminutive size, scarcely an inch 

 in length, very slender, and almost transparent. The body is 

 compressed laterally ; and there are no pectoral, ventral, anal, or 

 caudal fins, but only a single dorsal fin, extending the whole 

 length of the body. There are no eyes, nor any vestige of any 

 external organ, except a mouth, which is surrounded by small 

 tentacula, like that of the Hag. There is scarcely any trace of 

 a vertebral column, or of ribs ; yet the muscles are arranged with 

 great regularity, on the plan of those of Fishes in general. One 

 of the most curious parts of its structure is the complete absence 

 of cerebral hemispheres, and even of ganglia of special sense ; 

 the spinal cord being, apparently, the only centre of its nervous 

 system. Thus it may be characterised as one of those " expe- 

 riments prepared for us by Nature ;" exhibiting to us a case, in 

 which the Cerebrum is never developed ; the phenomena of which 

 closely correspond with the results that have been obtained 

 by the artificial removal of that organ. (ANIM. PHYSIOL. 

 465.) 



586. The Geological distribution of this class presents many 

 points of the greatest interest to the Zoologist. As might have been 

 anticipated from what is known of the history of the production 

 of the present crust of the globe, we find remains of Fishes in the 

 very earliest formations which distinctly exhibit the action of water, 

 that is, which were deposited as sediments in the bed of the 

 ocean ; and this long before we have reason to believe that any land 

 animals existed upon the surface of the globe. But the Fishes 

 of this early date were, for the most part, formed upon a very 

 different plan from those of the present epoch, so that there are 

 very few of those now existing, which bear any close resemblance 

 to them ; whilst, on the other hand, the greater proportion of 



