346 CHARACTERS AND AFFINITIES OF VERTEBRATES. 



ments. But it never becomes the backbone ; it is only the 

 internal scaffolding around which the backbone is built. The 

 backbone appears alike in history and in the individual as a 

 substitute for the notochord. For, from Cyclostomata 

 onwards, the external (mesodermic) sheath of the notochord 

 gains in importance, constricts and tends to obliterate what 

 it surrounds. In Fishes, and in some Amphibians and 

 Reptiles, part of the notochord persists in the adult, but in all 

 other cases the backbone, developed from the notochordal 

 sheath, almost entirely replaces the notochord. 



(3) Gill-Clefts. — From the pharynx of Balanoglossus 

 numerous respiratory slits open to the exterior ; in Appendicu- 

 laria and young Tunicates there is a pair, which persist in 

 the type named, but are replaced by numerous secondary 

 slits in the others ; in Amphioxus the slits are very numer- 

 ous; in Cyclostomata and Fishes they are limited to a 

 maximum of eight ; they are still to be seen and are used in 

 all young Amphibians, and also in those forms which 

 retain their gills even after they acquire the lungs which 

 all adult Amphibians possess. In higher Vertebrates, whose 

 embryos have an amnion and allantois, there are no longer 

 any gills, and the gill-clefts are wholly embryonic and virtually 

 functionless. In some cases they do not open. 



(4) Brain-Eye. — Many Invertebrates have well-developed 

 eyes, and those of cuttlefishes are very like those of Verte- 

 brates. But the eyes of Invertebrates arise in greater part 

 as insunk skin-cups, whereas those of Vertebrates are in 

 greater part outpushed vesicles of the brain. Yet this marked 

 difference is less than at first sight appears, since the hollow 

 brain is itself but an insunk tube of ectoderm. There is no eye 

 in Balanoglossus, nor in Amphioxus, nor in adult Tunicates. 



(5) Ventral-Heart. — Many Invertebrates have well- 

 developed hearts, but these are formed from a modification of 

 dorsal blood-vessels, whereas those of Vertebrate animals are 

 ventral. In Balanoglossus, however, what is sometimes 

 called a heart is dorsal. In Tunicates the heart is a very 

 simple tube, which drives the blood first in one direction 

 and then in another. In Amphioxus there is no definitely 

 developed heart, but there are many contractile regions on 

 the blood-vessels. From Cyclostomata upwards the heart 

 is well-developed. 



