IV. VERTEBRATA: PISCES. 



493 



Sub Class I. Myzontes {Hyperotrciia). 



Semiparasitic cyclostomes with cirri around the mouth, very primitive 

 nephridia, eyes rudimentary. From the large amount of mucus they are known 

 as sUme eels. They bore into fishes and eat the flesh. Myxine* east coast; 

 Bdellostovia* west coast. 



Siih Class. II. Petromyzontes (Hyperoartia). 



The lampreys {Peiromyzm*) have well-developed dorsal fins, and seven 

 gill openings. They occur in salt and fresh water, some marine species ascend- 



Fig. 544. — Petromyzon marinus* sea lamprey (after Goodc). 



ing streams to lay their eggs. The young have a larval (Ammoccetes) stage 

 with rudimentary eyes and slit-like mouth. Many species live on mucus and 

 blood which they rasp from fishes. 



Here may be mentioned a group of paleozoic fossils, the OSTRACO- 

 DERMI, of uncertain position. They have fish-like bodies, but no skeleton or 

 jaws are known. Ptcraspis, Cephalaspis, Pterichthys. 



Class II. Pisces (Fishes). 



The term fish is used in a wider and a narrower sense. In the first is 

 includes any acjuatic vertebrate swimming by means of fins and breathing 

 by gills; in the strict sense, as used here, it means aquatic branchiate forms 

 with vertebral column, cranium, and well-developed visceral skeleton; 

 with paired and unpaired fins, supported by a cartilaginous or bony 

 skeleton; with double nasal pits; with a skin and oral mucous membrane 

 which can produce ossifications, the scales and teeth. The fishes are the 

 best adapted of all vertebrates for an aquatic life, and their whole organ- 

 ization must therefore be considered from this standpoint. 



The epidermis consists of numerous layers of cells with an extremely 

 thin cuticle. Cornifications of this epidermis are lacking under ordinary 

 conditions, except a thin portion of the external subcuticular layer. At 

 the time of sexual maturity cornifications increase greatly in most 

 Cyprinoids and many Salmonids, producing hard bodies in the skin, the 

 pearl organs. Enormous numbers of large slime cells give the fishes their 

 well-known slippery skins. All protective structures arise from the 

 corium, which is composed of many layers of dense connective tissue and 



