868 TEXTBOOK OF ZOOLOGY 



an elaborate theory of the possible origin of the vertebrates from 

 this ancestry is derived. 



Echinoderm Theory. — This theory of vertebrate descent goes again 

 to Hemichordata. The developing egg of this animal becomes 

 a larva known as Tornaria, which floats in marine waters, has bi- 

 lateral symmetry, is almost transparent, and possesses bands of cilia 

 used in locomotion. This larva is almost exactly like that of the 

 starfish and other echinoderms which live in the same habitat. The 

 close correspondence of features of these two groups of larvae has 

 suggested the conclusion that these two types of animals have de- 

 scended from a common ancestor which was similar to these larvae. 

 The line of descent of one branch of this stock has presumably passed 

 through Hemichordata, Tunicata, Ampliioxus, and Vertebrata. The 

 nonchordate ancestors are not j^et conclusively determined, but the 

 foregoing theories suggest the thinking and evidence along that line. 



Within the subphylum Vertebrata the relations are somewhat more 

 evident, but the phylogenetic sequence is rather obscure at some 

 points. Cyclostomes, the simplest vertebrates, are most closely related 

 to Amphioxus, which has been suggested as the protochordate most 

 similar to vertebrates. These very primitive fish have an eel-like 

 body without paired fins and without jaws. They have from seven 

 to fourteen branchial (gill) apertures in different species, and all 

 of them possess skeletons composed of cartilage. The cartilaginous 

 skull is not entirely closed dorsally but resembles a trough with 

 bars over the roof. The anterior end of the nerve cord has expanded 

 to become a brain (Chapter XXVI). Next in order of complexity are 

 the Elasmobranchii, which possess well-developed, paired appendages 

 (fins) and jaws (Chapter XXVI). They also have a cartilaginous 

 skeleton, but the skull is much more complete dorsally. The number of 

 gill arches is reduced to five, but the apertures are uncovered as in 

 cyclostomes. The number of aortic arches has been reduced 

 from the sixty to ninety pairs of Amphioxus or seven pairs of 

 lamprej^ to five pairs. The group of ganoid fishes, which was the 

 dominant Devonian animal, is generally conceded to have Elasmo- 

 branch ancestry. Most ganoids have more or less cartilage along 

 with the bony structure of the skeleton. Their gills are covered 

 with an operculum, and there are only four aortic arches. The bony 

 ganoids are usually thought to be the ancestors of true bony fish. 



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