LAMPREYS. 65 



Class III. — CYCLOSTOMI. 



Scarcely larger in number of species than the last class is that which is called 

 either Cyclostomi, Cyclostomata, or Marsipobranchiata ; but when the details of 

 structure are considered, its members are at once seen to be worthy of classic rank, 

 exhibiting features which at once place. them near the base of the vertebrate series, 

 the lancelet and the tvmicates only being below them. Not only their structure and 

 their development proclaim them to be an ancient group, but in the Silurian rocks of 

 Europe and America teeth are found (called conodonts) which with considerable 

 probability ai"e regarded as indicating the existence of lampreys or hag-fishes in the 

 paleozoic seas. Owing to the entire absence of any other hard portions in their 

 anatomy, we cannot hope to find fossils of whose pertinence to this group there can 

 be no doubt. 



In shape the Marsipobranchs are much like eels, and in fact the species of JPetro- 

 myzon have among others the common name of lamprey eels. Structurally they 

 differ much from the true eels to be discussed further on. The notochord persists 

 through life, and the true skeletal elements are very feebly developed and never pass 

 beyond a cartilaginous condition. The vertebral column consists of a cartilaginous 

 sheath around the notochord (corresponding to the bodies of the vertebrse in the 

 higher vertebrates), and feebly developed dorsal and ventral arches. The skull also is 

 rudimentary, but serves to protect the comparatively simple brain. No true jaws or 

 paired fins are present. The gills are enclosed in a special pouch (whence the name 

 Marsipobranchii), and the water flows out through one or several external openings. 

 The mouth is nearly circular and is adapted for sucking. In order to hold on more 

 efiiciently, the mouth is furnished in the lampreys with numbers of sharp hook-like 

 teeth covered with a horny exterior. The alimentary canal is straight. There is but 

 a single nasal aperture, placed in the median line ; and according as its tube terminates 

 blindly or perforates the palate, the class Cyclostomi is divided into two orders. 



Order I. — HYPEROARTIA. 



In the lampreys the teeth are numerous, and are placed not only on the walls of 

 the mouth, but on the tongue as well. The nasal opening is in the middle of the 

 upper surface of the head, but it does not communicate with the mouth or throat. 

 From the pharynx a single tube on either side leads to the branchial chambers, where 

 the water passes over the gills and then out to the exterior through seven separate 

 holes on each side of the neck. No traces of gill arches are present. A spiral valve 

 is present in the intestine. Two eyes are present. The dorsal fin is usually divided 

 into two parts, one of which may be continuous with or separate from that which 

 terminates the tail. 



Some five or six genera and about twenty species of lampreys are known. They 

 frequent the seas and rivers of both the northern and southern temperate zones. The 

 names of the genera Petromyzon and Zampetra are derived respectively from the 

 Greek and the Latin, and both allude to a habit these animals have of attaching them- 

 selves to stones by their suctorial mouth. At other times they attach themselves in 

 VOL. in. —5 



