Cyclostomata. 153 



"connecting links" are represented only by fossil forms, 

 as, for example, the extinct animals that connect the 

 reptiles and the birds. 



The lancelets are plainly on the threshold of the verte- 

 brate household. By some authorities they are denied 

 admittance, and must wait just outside. Others allow 

 them barely to cross the threshold and humbly take their 

 place by the door, the lowest of the great branch at whose 

 head stands man. 



Distinction of the Lancelet from Other Vertebrates. On 

 account of the poorly developed brain and the absence of 

 a cranium, the lancelet is placed by itself in a division 

 called Acrania, while all the other vertebrates are desig- 

 nated as Craniata, from the presence of a skull and the 

 higher development of the brain. 



CLASS CYCLOSTOMATA. 



The lowest of the craniate vertebrates are the Cyclo- 

 stomata. This class includes the lampreys, or lamprey 

 eels, as they are often called, and the hagfishes. They 

 are eel-like in form, without scales, and with smooth, slimy 

 skins. They have no jaws, but a round, sucking mouth, 

 hence they are sometimes called the " round-mouthed eels." 

 There is a single nostril on top of the head. They have 

 dorsal and caudal fins, but no paired fins. There are sev- 

 eral pairs of purse-shaped gills, hence they are called by 

 some authors Marsipobranchii. The skeleton has no trace 

 of bone, being wholly cartilaginous and very imperfectly 

 developed. The internal organs are, in many features, 

 similar to those of the true fishes. 



Lampreys are rather widely distributed. They are 

 found along the Atlantic coast and ascend the rivers. 



