CLASSIFICATION 



641 



There is a primitive skull, but no true vertebrae (only bony arches). 

 Paired fins, true scales, and teeth are lacking. The gill-pouches are sac- 

 cular and the nose is unpaired. 



Sub-Class I. Myxinoidea (Fig. 366). 



These are the "hag-fishes" or "borers" which give off a slimy, 



Bdellostoma dombeyi (Pacific hagfish) 



3fyxinc r/lulinom (Atlantic lag fish) 



Pelromyzon marinns (sea lamprey) 

 Fig. 366. 



Cyclostomes. The light openings along the sides are mucous canals, the dark 

 ones are branchial openings. 



mucous jelly when captured . It is from this fact they receive their name 

 of Myxinoidea. 



Sub-Class II. Petromyzontia (Fig. 366). 



These are the lampreys, which live in both salt and fresh water. 

 The myxinoids attack principally dead and disabled fishes, but the 

 petromyzons attack decidedly active fish much larger than themselves, 

 attaching themselves to their host and making great inroads with their 

 rasping tongues. 



Class 2. Pisces (Gnathostomata). All fish having true lower jaws. 



Fishes are distinguished from the Cyclostomes not only by having 

 true lower jaws but also by having a vertebral column (amphicoele ver- 

 tebrae, Fig. 404), by having scales, paired pectoral and pelvic fins, and 

 paired nostrils. They breathe by gills and have a heart with .venous 

 blood therein only, although the heart has auricle, ventricle, sinus 

 venosus, and some have a conus arteriosus. 



Sub-Class I. Elasmobranchii. 



These are the sharks and their near relatives. They have a car- 

 tilaginous skeleton, usually a heterocercal tail, placoid scales (thornlike),, 

 but in Mustelus (the dog-shark, which is used in the laboratory), pointed,, 

 overlapping scales. There are five to seven slit-like gill-openings on 

 each side. The eggs are few and hatched within a sac inside the body. 

 The skates also belong to this group. They are merely flattened out 

 sharks. 



