AdO 



CHAPTER XII. 



FRESII-WATER FISHES OK THE SUB-CEASS CYCLOSTO.M ATA . 

 Gems Petkomyzov : Lanipr(>y — Tiampcrn — Mud Lamprey, or Pride. 



Genus : Petromyzon (Artedi). 



The fishes included in this g-enus beloni^ to a distinct Division of the Class 

 Fishes; and ai'e placed in an Order named Marsijjofjranchii. The body is 

 Eel-like or worm-like in form, and under<yoes a remarkable metamorphosis. 

 The skin has neither scales, nor scutes, nor lateral line, though there are a few 

 scattered pores upon the head. The eyes are feebly devek)ped in the larval 

 stage of existence, though they subsequently acquire a more perfect and 

 larger condition. There is but one nasal aperture, in the middle line of the 

 upper part of the head. The mouth is uuctorial ; and when open is circular 

 and seen to be tilled with a large number of horny denticles, yellowish-brown 

 in colour, and arranged in rows. When closed the mouth is a longitudinal 

 furrow. The animal adheres by suction to various substances and to fishes, 

 and thus has a semi-parasitic character. There are consequently no true jaws; 

 and the modifications of the head due to its suctorial habit, have led Professor 

 W. K. Parker to compare it with the tadpole of the frog, and even to regard 

 it as more nearly related to such a type than to fishes or other vertebrata. 

 The skeleton is cartilaginous and remains in an embryonic condition. The 

 notochord not being divided, the vertebrae are not separated from each other, 

 though their arches are distinctly developed. The skull is cartilaginous; 

 but the brain is formed in the way usual in fishes, and gives off the usual 

 nerves. There are seven branchial openings for the gills, and the gill sacs 

 are supported by a complicated cartilag-inous framework, in which the branched 

 lateral arches are connected tog-ether at the sides, so as to surround the gill- 

 openings ; and they are also connected below in the median line, forming" 

 a structure termed the extra-branchial basket, which terminates behind in a 

 transverse convex cartilage which is defined as extra-pericardial. 



'V\\Q alimentary canal is straight and simple, and terminates in a spiral 

 valve. The heart contained in a pericardium has its several parts divided by 

 valves, and resembles the heart of a fish of the ordinary type. The reproductive 

 organs are single, there being but one ovary or one milt in each individual. 



