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 Actuctary sac formed of the 
enlarged Aypophysis. The hypophysis usually opens into the 
stomodzeum 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.— Fetromyzontes. 
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 vertebree. 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 cesophagus. 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 larve. The larva is known as Ammocetes. It 
differs from the adult in several important particulars, e.¢., the 
M. 29 
