, 



TUB LOWEU VEUTEBRATA. 209 



Next, what are the main points which separate off 

 Amphioxus from the higher types? The latter all possess 

 (with odd exceptions here and there) : 



(1) An epidermis of more than one layer. 



(2) Skull and vertebrae. 



(3) Two paired tins or limbs. 



(4) Paired eyes, with the sensory elements of the retina 

 turned away from the light. 



(5) Paired ears, with semicircular canals. 



(0) Paired olfactory organs. 



(7) Sympathetic ganglia. 



(8) A heart. 



(9) A spleen. 



(10) A compact liver. 



(11) Compact kidneys. 



None of these are found in Amphioxus. On the other 

 hand, Amphioxus has 



(1) Very numerous gill-slits. 



(2) An atrium. 



(3) Metameric gonads. 



(4) Numerous metameric nephridia. 



Thus we must divide the Vertebrata (Chordata) into 

 two * great groups (or rather one great and one small) 



A. AcRANiA,f including the Amphioxus only. 



B. CRANIATA (VEKTEBRATA of some authors), which in- 

 cludes fishes, amphibia, reptiles, birds and mammals. 



* The Turiicata form a third, and certain other forms may be put 

 into a fourth. 



f An astonishing number of other names have been bestowed on 

 this group by various authors : the only one in sufficiently common 

 use to be worth mentioning is that of Ceplialoclwrda, referring to the 

 forward extension of the notochord. 



Questions on the Vertebrata. 



[Most of those questions were set at London University 

 Examinations.] 



1. Give an account, with illustrative sketches, of the 

 digestive organs of the common frog, specifying particularly 

 the different forms of epithelium met with in the several 

 regions thereof. 



ZOOL. 14 



