IV. VERTEBRATA: PISCES. 557 



and choroid lacking). From the large amount of mucus they are known 

 as slime eels. They bore into fishes and eat the flesh. Myxine* on the 

 east coast, Bdellostoma * (Polistotrenid) on the west. 



Sub Class II. Petromyzontes (Hyperoartia). 



Several American species of lampreys, all belonging to Petromyzon* 

 (with sub genera), have well-developed dorsal fins, and seven branchial 

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



FIG. 586. Petromyzon marinus,* sea lamprey. (After Goode.) 



ascending streams to lay their eggs. The young pass through a larval 

 (Ammoccetes) stage with rudimentary eyes and slit-like mouth. Many of 

 the species live on the mucus and blood which they rasp from fishes. 



Here may be mentioned a group of fossils, the OSTRACODERMI, of 

 uncertain position. They have fish-like bodies, but no skeleton or jaws are 

 known. They flourished in paleozoic seas. Pteraspis, Cephalaspis, 

 Ptericlitliys. 



Class II. Pisces (Fishes). 



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

 first it includes any aquatic vertebrate swimming by means of fins 

 and breathing by gills; in the more strict sense, as used here, it 

 means aquatic branchiate forms with vertebral column, cranium, 

 and well-developed visceral skeleton; with paired as well as 

 unpaired fins, these supported by a cartilaginous or bony skeleton 

 in addition to horny rays; with double nasal pits; with a skin and 

 oral mucous membrane which can produce ossifications, the scales 

 and teeth. The cyclostomes are thus excluded. The fishes are 

 the best adapted of all vertebrates for an aquatic life, and their 

 whole organization must therefore be considered from this stand- 

 point. 



The epidermis consists of numerous layers of protoplasmic cells 

 with an extremely thin external cuticle. Cornifications of this 

 epidermis are lacking under ordinary conditions, with the excep- 

 tion- of 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, 



