206 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 Miiller, as modified above, and which wc shall 

 for the most part adopt. The subject oi 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. 



Amphioxid.i^.. This family, at present represented by the single genus 

 Bj-anchiostoma, occupies the lowest position in the scale of fishes. It is 

 characterized by the naked skin, the single dorsal extending over the entiie 

 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 

 embrvonic 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 BrancJiiostoma 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. 



Myxinoidei. The Myxinoids are hardly more fish-like in their appear- 

 ance than the Branchiastoma, having been classed, by Linnaeus and other 

 writers, among the worms. The vertebral column, as in Branchiostoraa, 

 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. Myxine 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 

 No^tth America. 



PETROMVzoNTiDiE. This family, tlic last of the Dermopteri, 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 Ammocates, or 

 sand lamprey. The former have a circular mouth provided with numerous 

 teeth, and fringed with cilise 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 

 4;.o 



