CYCLOSTOMI. 155 



lips and often by filamentous processes, is circular in shape, though 

 the lips can be applied together so as to form a median longitudinal 

 slit. It leads into a funnel-shaped buccal cavity, which is without 

 jaws and is armed on the soft palate as well as on the floor with 

 horny teeth (fig. 601). At the bottom of the funnel is the tongue, 

 which, moving up and clown like a piston, enables the animal to attach 

 itself by its mouth as by a sucker. The pharynx, which follows 

 the mouth, communicates with the branchial sacs either directly or by 

 a special passage (Petromyzon). The intestinal canal passes straight 

 to the rectum and is divided into stomach and intestine by a narrow 

 region, the walls of which project so as to form a sort of valve. The- 

 liver is always well developed, but there is 

 no swimming bladder. 



The gills (fig. 592) lie at the sides of the 

 oesophagus in six or seven pairs of branchial 

 sacs. These open on either side by external 

 branchial passages into the same number of 

 separate respiratory apertures. In Myxine 

 on the other hand there is on each side, almost 

 on the ventral surface, only one opening, into 

 which all the external branchial passages of 

 the same side open. 



On the other side the sacs communicate 

 with the oesophagus, but, except in Ammo- 

 ccetes, never directly by simple openings but 

 by internal branchial passages or. as in Fi. eoL Head of Pafro*** 



marinus, seen from below 



Petromyzon, by a common passage lying showing the homy teeth 

 beneath the oesophagus into which passage J the buccai a ^ ty (after 



Heckel and Kner). 



all the other branchial passages open. The 



water flows in from the exterior through the external branchial 

 openings or in Myxine through the nasal passage, and is driven by 

 the contraction of the constrictor muscles of the branchial sacs either 

 out by the same way (Petromyzon), or into the oesophagus, and from 

 this to the exterior through a special unpaired canal on the left side. 



The heart lies beneath and behind the branchial skeleton. Some 

 of the vascular trunks pulsate, e.g., the portal vein in Myxine. The 

 aortic bulb has no muscular layer, and contains, as in the Teleosteans, 

 only two valves. 



The urinary and genital organs are of simple structure. In 

 Myxine the kidneys retain the primitive segmental arrangement, 

 there being a urinary tubule and Malpighian body in every seg- 



