3 o MORPHOLOGICAL TYPES 



in front into the buccal cavity guarded by the remnants of the 

 velum. 



These changes which take place during the life of Petro- 

 myzon show that it is a form descended from ancestors which 

 practised the ciliary mode of feeding, since when, it developed 

 a specialised and somewhat degenerate method of feeding of 

 its own. 



Behind the pharyngeal region the gut runs straight as the 

 intestine to the anus : there is no indication of a curved and 

 enlarged region known in all higher forms as the stomach. 

 In the intestine the surface of absorption is slightly increased 

 by a small inwardly projecting ridge which, as it winds heli- 

 coidally down the intestine, is known as a " spiral valve." 

 The lining of the gut is ciliated. 



Ventral outgrowths from the front of the intestine form the 

 liver. It is more or less degenerate in the adult, and it is said 

 that its communication with the intestine by means of the 

 bile-duct becomes lost, so that its only communication is with 

 the blood-vessels. The pancreas is very rudimentary, and 

 represented only by scattered packets of cells along the 

 intestine. 



Vascular System. — Running forwards beneath the intestine, 

 and therefore in the splanchnopleur, is a subintestinal vessel. 

 It runs to the liver where it breaks up into capillaries forming 

 a hepatic portal system. From the liver, the vessel proceeds 

 forwards as the hepatic vein, and soon swells out beneath the 

 pharynx and becomes specialised to form a muscular pump : 

 the heart. 



The heart is composed of the following structures : a sinus 

 venosus, into which the hepatic and other veins enter ; leading 

 on to a thin- walled auricle and a thick- walled ventricle. The 

 entry to and exit from the ventricle, which does the propelling 

 of the blood, are guarded by valves so that blood cannot flow 

 in the reversed direction. The length of the structures com- 

 posing the heart is greater than that of the space (pericardium) 

 in which they lie ; consequently the heart is slightly bent on 

 itself into the form of an S. 



From the ventricle the ventral aorta runs forward beneath 



