EVIDENCE FROM EMBRYOLOGY 65 



Dipnoi or lung-fishes, the air-bladder is utilized for 

 purposes of respiration. 



It has been objected that, while embryology may 

 prove relationship within a single type, it fails to 

 demonstrate any connection between different types, 

 but this is not altogether true. The Tunicata, a 

 curious group of marine animals once referred to the 

 Mollusca, are shown by their ontogeny to be related 

 to the vertebrates and the same is true of certain 

 marine worms (Balanoglossus) . Indeed, most mod- 

 ern zoologists have adopted a scheme of classifica- 

 tion, in which the type Chordata includes not only 

 the true vertebrates, but also the Lancelet (Am- 

 phioxus), the tunicates and Balanoglossus; this 

 scheme is founded upon the embryological evidence. 

 Among the invertebrates even more remarkable ex- 

 amples have been observed. Such radically differ- 

 ent types as the segmented worms and the shell-fish 

 (Mollusca) are brought into relationship by their 

 ontogeny and their closely similar types of larvae, as 

 are also, though less distinctly, the brachiopods or 

 lamp-shells, and the Bryozoa. The Horseshoe-crab, 

 or King-crab, so abundant along our Atlantic coast, 

 was long of uncertain affinities; originally referred to 

 the Crustacea, largely because of its marine habits 

 of life, embryology makes much more probable 

 its relationship to the air-breathing scorpions and 

 spiders, a result which will be examined again from 

 another point of view in connection with blood-tests. 



Even before the publication of Darwin's "Origin of 



