96 



SUB-CLASS MARSIPOBRAXCHII (CYCLOSTOMATA). 



Petromyzontidae and in the tail of myxinoids the fins are sup- 

 ported by cartilaginous somactids. They possess a suctorial 

 mouth, which is without jaws, but is provided with horny teeth. 

 By means of it, with the assistance of a suctorial tongue-like 

 structure they attach themselves to and suck their prey. 

 Myxine indeed bores its way into the body cavity of other fishes, 

 and is truly parasitic. The nasal aperture is single, and leads 

 into an unpaired nasal sac. In this and in other features of 

 their anatomy, which will be described later, they are unique 

 amongst Vertebrates. Nevertheless, we shall not follow the 

 example of some zoologists who have established the Marsipo- 

 branchii as a separate class of the Vertebrate, distinct from the 

 class Pisces. We hold them, in spite of the remarkable and 

 unique features of their organization to be true Pisces, not only 



FIG. 48. a. Petromyzon fluviatilis (after Heckel and Kner). b, c, d, stages in the transforms' 

 tion of Ammocoetes branchialis into Petromyzon planeri (after v. Siebold) ; b head of an eye- 

 less larva, side view ; c the same, ventral view ; d later stage with small eyes, side view. 



by their aquatic habit of life, but by the characters of their 

 respiratory and vascular organs. They possess a simple tubular 

 heart, which distributes the blood by means of a ventral aorta 

 to the walls of the gill pouches ; and these open to the exterior 

 on the sides of the body in the ordinary piscine manner. In 

 the structure of their mouth parts they present some resemblance 

 to the larvae of anurous Amphibia, but the resemblance is too 

 vague to permit of any definite approximation to that group 

 in classification. 



The Marsipobranchii fall into two main groups which present 

 marked points of difference from one another. These are the 

 Petromyzontidae or lampreys, and the Myxinidae or hag-fishes. 

 In the Petromyzontidae the nasal sac does not communicate with 

 the mouth, the eyes are normally developed, and possess eye- 

 muscles with their corresponding nerves, the pericardium does 



