24 VERTEBRATE ANIMALS OF THE UNITED STATES 



Internal Structures. — A knowledge of the internal anatomy of a 

 lish is usually not necessary in order to make a systematic description 

 of it. The following features, however, are occasionally referred to 

 in the descriptions in this book: the shape of the stomach; the number 

 and character of the pyloric appendages or caeca of the stomach; the 

 length and character of the intestine; the presence or absence of the air 

 bladder, and its character when present; the character of the four ante- 

 rior vertebrae, whether they are coossified and have some of their ele- 

 ments detached to form the Weberian ossicles which connect the air 

 bladder with the ear; the unossified portion of the top of the skull in 

 certain fishes known as the fontanelle. 



Key to the Subclasses of Fishes (including the Cyclostomes) 



El Mouth without jaws; no paired fins; nostrils median i. Marsipobranchii. 



a2 Mouth with true jaws; paired fins present; nostrils paired. . 2. Pisces. 



Subclass I. Marsipobranchii {Cyclostomata) . — Fish-Hke verte- 

 brates with a cylindrical, eel-like body, without scales, paired tins, gill- 

 arches or jaws; skeleton cartilaginous and without ribs; skull imperfectly 

 developed and not separate from the vertebral column; nostril single, 

 median; mouth suctorial; gills sac-like, 6 to 14 in number, opening to the 

 outside through paired pores; no genital ducts, sympathetic nervous 

 system, arterial bulb, pancreas, spleen or air bladder; median fins on 

 posterior half of body: about 20 species, mostly parasitic on fishes in 

 both fresh and salt water, grouped in 2 orders. The first of these orders, 

 with 2 American species, the Californian hagfish, Polistotrema stouti 

 (Lockington) and the Atlantic hagfish, Myxine glutinosa L., is exclu- 

 sively marine. 



Key to the Orders of Marsipobranchii 



ai Gill openings not near head; hagfish, aU marine, and not included 



in this book i. Hyperotreti. 



Z.2 Gill openings immediately back of head; lampreys 2. Hyperoartii. 



Order 2. Hyperoartii. — The lampreys. Body cylindrical ante- 

 riorly, compressed posteriorly; fins median and well developed, the dorsal 

 fin being more or less completely separated by a median notch into 

 two fins; nostril just in front of the eyes and opening into a blind sac 

 which does not communicate with the pharynx; eyes well developed 

 in the adult; mouth suctorial, in the middle of a depressed, funnel- 

 shaped oral plate, called the buccal funnel (Fig. 10), which has a fringed 

 margin and is set with sharp, horny teeth or toothlike tubercles; tongue 



