266 THE STUDY OF ANIMAL LIFE CHAP. 



of the body, forming the brain and spinal cord. These 

 arise by the infolding of a skin groove on the dorsal mid- 

 line of the embryo, (b) Underneath the nerve-cord in 

 the Vertebrate embryo is a supporting rod or notochord. 

 It arises along the roof of the food-canal, is therefore of 

 endodermic origin, and serves as a supporting axis to the 

 body. It persists in some of the lowest Vertebrates (e.g. 

 the lancelet) ; it persists in part in some fishes, e.g. mud- 

 fishes ; but in most Vertebrates it is replaced by a meso- 

 dermic growth the backbone which ensheaths and 

 constricts it. (c) From the anterior region of the food- 

 canal in fishes and tadpoles, slits, bordered by gills, open 

 to the exterior. Through the slits water flows, washing 

 the outsides of blood-vessels and aerating the blood. 

 These slits or clefts occur on the neck-region of all 

 young Vertebrates, but in reptiles, birds, and mammals 

 they are transitory and never used, except that the 

 first one becomes the Eustachian tube leading from the 

 ear-passage to the back of the mouth. Amphibians are 

 the highest animals in which gill-slits are used for breath- 

 ing, and even then they may be entirely replaced by 

 lungs in adult life. They are evident in tadpoles, they 

 have disappeared in frogs, (d) Many an Invertebrate 

 has a well-developed heart, but this always lies on the 

 dorsal surface of the body, while that of fish or frog, bird 

 or man, lies ventrally. (e) It is characteristic of the eye 

 of backboned animals that the greater part of it arises 

 as an outgrowth from the brain, while that of backbone - 

 less animals is directly derived from the skin. But this 

 difference is less striking when we remember that it is 

 from an infolding of skin that the brain of a back- 

 boned animal arises. 



But while the characteristics of backboned animals can 

 be stated with some precision, it is not possible to draw 

 with a firm hand the dividing line between backboned 

 and backboneless. Thus fishes are not the simplest 

 Vertebrates ; the lamprey and the glutinous hag belong 

 to a more primitive type, and are called fishes only by 

 courtesy ; simpler still are the lancelets ; the Tunicates 

 hesitate on the border line, being tadpole-like in their 



