PH ARYNGOBR ANCHII. 353 



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 lanceolattis, or Lancelet, the organ- 

 isation 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 notochord is persistent ; and there are no verte- 

 bral centra nor arches. No distinct brain nor auditory organs 

 are present. In place of a distinct heart, pulsating 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 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 

 ventral surface, and expands at the tail to form a lancet-shaped 

 caudal fin. No true paired fins, representing the anterior and 

 posterior limbs, are present. The mouth is a longitudinal 

 fissure, situated at the front of the head, and destitute of 

 jaws. It is surrounded by a cartilaginous ring, composed of 

 many pieces, which give off prolongations, so as to form a 

 number of cartilaginous filaments or " cirri " on each side of 

 VOL. n. z 



