224 CLASSIFICATION OF VERTEBRATES. 



SUB-CLASS II. MYXINOIDEI (HYPEROTRETIA). 



Dorsal fin small or absent, hypophysial duct opening at tip 

 of snout, posteriorly communicating with mouth cavity. Bran- 

 chiae 6-14 pairs, the pharyngeal region not divided. Behind 

 the gill clefts on the left side occurs an oesophageo-cutaneous 

 duct, connecting the pharynx with the exterior. Branchial bas- 

 ket rudimentary, snout bearing eight barbels ; spiral valve absent. 



The hag-fishes and borers are the nearest approach to para- 

 sites in the vertebrate phylum. They fasten themselves to the 

 gill region of fishes, and work their way into the inside, where they 

 rapidly devour the flesh, leaving merely a hulk of skin and bones. 



Family Myxinid.e. BranchiK 6, the clefts uniting external to the 

 branchial pouches to open to the exterior by a single opening. Myxine, 

 the only genus. J/, glutinosa, in the northern Atlantic south to Cape Cod. 

 Family Bdellostomid.e. Gills 6-14, each with its own duct. Bdellostoma 

 {Heptatrema, Polistotrema), the only genus, occurs in tlie Pacific, where 

 £. dombey ranges from California to Chile. 



OSTRACODERMI. 



The ostracoderms are a group of fish-like forms from palaeozoic rocks 

 cf exceedingly doubtful relations. They have been regarded as ganoids, 

 as cyclostomes, as tunicatos, and even as having relations to the xiphosures 

 {Limulus). This uncertainty of position is due to the fact that no traces 

 of jaws or of internal skeleton have yet been found. The anterior half of 

 the body was enveloped in an exoskeleton of large bony plates which no 



Fig. 229. Restoration of Pleraspis, after Lankester. 



one has yet satisfactorily homologized with the bones in any modern form. 

 The hinder part of the body was fish-like, the tail heterocercal. Frequently 

 the anterior plates bear traces of canals supposed to have contained lateral 

 line organs, while the head region contained pits which may have been 

 occupied by eyes. 



ORDER I. HETEROSTRACI. 



Head region covered above by a few firmly united plates, below by a 

 single ventral plate ; tail sometimes scaled ; orbits lateral ; no paired appen- 

 dages. Pieraspis, the best-known genus, comes from the De\onian, 



