CHAP, xvi Backboned Animals 249 



is a supporting rod or notochord. It arises along the roof of the 

 food-canal, and serves as a supporting axis to the body. It per- 

 sists in some of the lowest Vertebrates (e.g. the lancelet) ; it persists 

 in part in some fishes ; but in most Vertebrates it is replaced by a 

 new 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 are represented in the young of all 

 Vertebrate animals, but in reptiles, birds, and mammals they are 

 transitory and never used. Amphibians are the highest animals in 

 which they are used for breathing, and even then they may be 

 entirely replaced by lungs in adult life. They are evident in tad- 

 poles, 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 backboneless 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 backboned animal arises. 



But while the characteristics of backboned animals can now be 

 stated with a precision greater than that of sixty years ago, it is no 

 longer 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 

 is the lancelet ; the Tunicates hesitate on the border line, being 

 tadpole-like in their youth, but mostly degenerate when adults ; 

 and the worm-like Balanoglossus is perhaps to be ranked as an 

 incipient Vertebrate. The extension of knowledge and the appli- 

 cation of evolutionary conceptions obliterate the ancient landmarks 

 of more rigid but less natural classification. 



I. BalanoglosSUS. Balanoglossus is a worm - like animal, 

 represented by some half-dozen species, which eat their way 



FIG. 50. Balanoglossus, showing proboscis, collar, and gill-slits. 



through sandy mud off the coasts of the Channel Islands, Brittany, 

 Chesapeake Bay, and other regions. Its body is ciliated and divided 

 into distinct regions a large "proboscis" in front of the mouth, a 



