206 7 ZOOLOGY. 
We come in the next place to the consideration of the more prominent 
of the families above mentioned ; taking them up in the order in which 
they occur in the system of Miller, as modified above, and which we shall 
for the most part adopt. The subject of Ichthyology is of such vast extent, 
that our limits would be transgressed even by mentioning the names alone 
of all the species. We can only refer particularly to the species distin- 
guished by their economical value or physiological interest. . 
Ampuioxip&. This family, at present represented by the single genus 
Branchiostoma, occupies the lowest position in the scale of fishes. It is 
characterized by the naked skin, the single dorsal extending over the entire 
length of the back, the absence of pectorals and ventrals. Mouth entirely 
inferior, elongated, or circular, the margins provided with a row of free 
filaments. Anterior to the anus is a median fin representing the transition 
embryonic fin of higher fishes. The brain, as an expansion of the nervous 
centres, is entirely absent ; being represented by a simple extension of the 
spinal cord, which runs out to a blunt point. The absence of brain and of 
heart sufficiently distinguishes Branchiostoma from all other fishes. Its 
aspect, in fact, is hardly that of a fish at all, being highly vermiform in 
appearance. The species hitherto described are all from European or 
North African waters. 
Myxinower. The Myzxinoids are hardly more fish-like in their appear- 
ance than the Branchiostoma, having been classed, by Linnezeus and other 
writers, among the worms. The vertebral column, as in Branchiostoma, 
exhibits the persistence of the usually temporary chorda-dorsalis. The 
mouth is terminal, nearly circular, and provided with a powerful hooked 
tooth on the palate, with others in the tentaculated mouth. The lateral 
fins are entirely absent, as in the other Dermopteri. The species of this 
family, though small in size, are formidable from the manner in which they 
attack their prey, which consists of larger fish. Myzxine glutinosa, a 
European form, is most generally found embedded in the bodies of other 
fish, as the cod, haddock, and others. The manner of its entrance is not 
exactly known; probably, however, by a perforation made by the mouth, 
assisted by the powerful hook. No species of this family are found in 
Noxth America. 
Perromyzontip&. ‘This family, the last of the Dermoptert, is also without 
lateral fins: a continuous median fin is formed by the coalescence of the 
dorsal, caudal, and anal. Respiration is generally performed by means of 
fixed gills, the lateral openings to which are seven in number on each side. 
A single nostril is placed on the top of the head. The principal forms 
belong to the genera Petromyzon or true lamprey eel, and Ammocetes, or 
sand lamprey. The former have a circular mouth provided with numerous 
teeth, and fringed with cilie to assist the animal in attaching itself to the 
bodies of its prey. The mouth is a true sucker, adhesion being effected by 
atmospheric pressure. Fishes of various kinds are not unfrequently caught 
bearing the bloody circular scar produced by the bite of the lamprey, and 
quite often the lamprey itself. The catfish, or Pimelodus, appears to be 
especially liable to such attacks. The lampreys attain to great size, and 
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