400 



DIVISIONS OF FISHES. 



CHAPTER LV. 



PHARYNGOBRANCHII AND MARSIPOBRANCHIL 



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. 169), 

 the organisation of which differs in almost all important points 

 from that of all the other members of the class. 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 notocliord is persistent ; and there are no 

 vertebral centra nor arches. No distinct brain nor auditory 

 organs are present. In place of a distinct heart, pulsating dila- 

 tations 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 fissures, the 

 sides of which are ciliated, the whole exercising a respiratory 

 function. 



The Lancelet is a singular little fish which is found burrow- 

 ing in sandbanks, in various seas, but especially in the Medi- 

 terranean. The body is lanceolate in shape, and is provided 

 with a narrow membranous border, of the nature of a median 

 fin, which runs along the whole of the dorsal and part of the 



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

 terygii(or Cartilaginous Fishes), the A canlhopterygii (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, Cycloidei, Ctenoidei, Ganoidei, and Placoidd. 

 Miiller divides the fishes into the five orders Leptocardia (Lancelet), Cyclo- 

 stomata (Lampreys and Hag-fishes), Teleostd (Bony fishes), Ganoidei 

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



