INVERTEBRATA. 447 



of the Urochorda or Tunicata exhibit the process of gem- 

 mation, forming compound gemmaria as in other sessile 

 animals. One class of the Urochorda is pelagic, that is, 

 adapted to a free swimming life at the surface of the sea. 



CLASS : CEPHALOCHORDA. 



The characters of this class are those of Amphioxus, 

 for the differences between the various genera and species, 

 which are few in number, are only in small details. 

 Cephalochorda differ from the other Prochordata in the 

 elongated fish-like form, in the fact that the notochord 

 extends to each extremity of the body, and in the special 

 relations of the atrium. 



SUB-PHYLUM VERTEBRATA OR CRANIATA. 



Since all the members of this group possess some 

 rudiments of vertebrae if it be only as in the Lamprey a 

 few neural or haemal arches without centra, there seems 

 no reason why the old name Vertebrata should not be 

 retained. Some authorities, however, regard the skull as 

 more characteristic and therefore prefer the name Craniata. 

 Other common characters are the well-developed brain 

 with its common plan of structure, the well-developed 

 heart, the portal system and other general features of the 

 vascular system, the urogenital system. In fact all the 

 Vertebrata are modifications of a common fish-like 

 ancestor. 



The Lampreys (Petromyzori) and Hag-fishes (Myxinoids) 

 form a group which has been placed in various positions in 

 the classification of the Vertebrata. These animals have 

 no paired fins, but have fin-rays in the median fin ; they 

 have gill-slits which take the form of pouches, communi- 

 cating with the exterior and with the pharynx by tubular 

 ducts. Accordingly they are considered by Huxley as an 

 order of the class of fishes. The fact, however, that the 

 mouth is suctorial and possesses no movable jaws, and that 

 the pituitary sac opens permanently outside the mouth, 

 with the nasal sac single and opening into the pituitary 

 sac, differentiates these animals from all other fishes and 

 therefore later authorities have made them a separate 



