477 



DIVISIONS OF FISHES. 



CHAPTER LII. 







PHARYNbUBJiANCHII AND MARSIPOBRANCH1L 



THE class Pisces has been very variously subdivided by dif- 

 ferent writers ; but the classification here adopted is the one 

 proposed by Professor Huxley, who divides the class into the 

 following six orders, in the subdivisions of which Professor 

 Owen has been followed : * 



ORDER I. PHARYNGOBRANCHII ( = Cirrostomi, Owen ; and 

 Leptocardia, Miiller). This order includes but a single fish, 

 the anomalous Amphioxus lanceolatus, or Lancelet (fig. 259), 

 the organisation of which differs in almost all important points 

 from that of all the other members of the class. The charac- 

 ters of Amphioxus, in fact, are so aberrant, that Haeckel pro- 

 poses to divide the sub-kingdom Vertebrata into two primary 

 sections the one (Leptocardia] comprising the Lancelet alone, 

 whilst the other (Pachycardia) includes all other Vertebrates. 

 The order is defined by the following characters, which, as will 

 be seen, are mostly negative : No skull is present, nor lower 

 jaw (mandible), nor limbs. The notochord is persistent ; and 

 there are no vertebral centra nor arches. No distinct brain nor 

 auditory organs are present. In place of a distinct heart, pul- 

 sating dilatations are developed upon several of the great blood- 

 vessels. The blood is pale. The mouth is in the form of a 

 longitudinal fissure, surrounded by filaments or cirri. The 

 walls of the pharynx are perforated by numerous clefts or fis- 

 sures, the sides of which are ciliated, the whole exercising a 

 respiratory function. 



* Cuvier divided the class Pisces into the great orders of the Chondrop- 

 ierygii(or Cartilaginous Fishes), the Acanthopterygii (or Fishes with spinous 

 rays in the paired fins), and Malacopterygii (or Fishes with soft rays 

 in the paired fins). Agassiz divides Fishes, from the character of the 

 scales, into the four orders, Cycloidd, Ctenoidei, Ganoidei, and Placoidei. 

 Miiller divides the Fishes into the five orders Leptocardia, (Lancelet), Cydo- 

 stomata (Lampreys and Hag-fishes), Teleostei, (Bony Fishes), Ganoidei, 

 (Ganoid Fishes), and Selachia (Sharks and Rays). 



