CYCLOSTOMATA. 433 



suctorial mouth and by a branchial basket-work. There is 

 no vertebral column, and at most an incomplete cartilaginous 

 cranium. In accordance with this the notochord and its 

 thickened sheath form the skeletal axis throughout life. 



The olfactory sac is single and does not open directly 

 to the exterior, but into a long pituitary sac formed of the 

 enlarged hypophysis. The hypophysis usually opens into the 

 stomodaeum from which it originates, but in the course of 

 development (in the lamprey) a large upper lip is formed 

 behind the opening of the hypophysis, and pushed out to 

 such an extent as to carry the base of the hypophysis on to 

 the dorsal surface of the head. As we have seen the 

 hypophysis acquires an internal opening into the pharynx in 

 Myxine but not in the Lampreys. The olfactory capsule is 

 free from the cranium. 



The auditory organ never has three semi-circular canals. 

 The brain shows a very small cerebellum and is of small 

 proportionate size. The optic nerves do not cross, and 

 there is no sympathetic nervous system nor spleen. 



In some forms the pronephros persists throughout life, 

 the tubules opening into the pericardium. There are no 

 genital ducts, the sexual elements leaving the coelom by 

 pores. 



Order I. — Petromyzontes. 



The Lampreys are active free-swimming forms with pre- 

 datory habits. They have dorsal fins, well-developed eyes, 

 and have two semi-circular canals to the ear. The pituitary 

 sac is blind. The skeleton is a slight advance upon that of 

 Myxine as there are paired lateral nodules of cartilage 

 representing vertebrae. There is also a complete branchial 

 skeleton, but no buccal cirri nor cartilages. 



The seven gill-pouches open separately to the exterior 

 laterally, and internally they all open into a respiratory tube 

 which communicates anteriorly with the oesophagus. The 

 intestine contains a spiral valve. The sexes are distinct. 



The Lampreys are widely distributed in the sea and in 

 fresh water. They develop by an early embryonic stage 

 and later larvae. The larva is known as Ammocoetes. It 

 differs from the adult in several important particulars, e.g.^ the 



M. 29 



