92 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



0. Teleostoidei ; * represented by the North 

 American bow-fin (Anna), and gar-pike (Lepidosteus), 

 and the North African Polypterus ; in these the 

 hinder part of the cartilaginous cranium always under- 

 goes ossification, the spiracles close up, or are covered 

 by a bony plate, and the scales, which are never 

 formed of true bone, are large, and may be covered by 

 a layer of enamel. Vertebrae are developed around 

 the notochord. 



The Teleostei, or bony fishes, always have ossi- 

 fied vertebral centra, and more or less of the primitive 

 cranial cartilage is finally replaced by bone ; scattered 

 bony plates are developed in the dermis, or the in- 

 tegument is protected only by thinner scales, or the 

 body is naked ; they are divisible into : 



a. Physostomi, where the air bladder, which is 

 an outgrowth of the oesophagus, almost always remains 

 connected with it by an open duct, and the hinder 

 pair of fins, if retained, as in the salmon, are always 

 abdominal in position ; here we find catfishes (Silurus), 

 carps, pikes (Esox), and salmons, as well as the finless 

 eels. 



. Pliysoklisli. In these the air bladder be- 

 comes shut off from the oesophagus, or is aborted, as 

 in the sole ; the ventral fins, which are rarely ab- 

 dominal (Notacanthus), are ordinarily thoracic or 

 jugular in position ; not unfrequently they are rudi- 

 mentary or lost. The fin-rays are either all jointed 

 as in the cod, or some are entire, as in the perch. 

 Some forms are asymmetrical and flattened like the 

 sole ; some swollen and globular like the sun-fish ; some 

 greatly elongated like the pipe-fish; some with a 

 prehensile tail like the sea-horse ; some have the body 

 scaleless ; others, like Diodon, have erectile spines ; 

 some can live in semi-fluid mud (Ophiocephalus) ; 



* Holostei. 



