536 



HISTORY OF THE HUMAN BODY 



one possible way out, a single remaining clew, and that is, 

 the embryology of the form in question, and even here the 

 primary, historic features may be overlaid with secondary 

 changes rendered necessary as an adaptation, and thus the 

 value of a given feature is often hard to estimate. In the 

 case of Balanoglossus, however, it seems probable that the early 

 development is in great part an actual repetition of the race- 

 history, but if so, it leads us to surprising and not very satis- 

 fying results, for the animal begins life as a minute transparent 

 floating larva, the Tornaria-) furnished with bands of cilia, by 

 which it moves, a larva strikingly like that of star-fish, sea- 

 urchins, and other echinoderms, and one which an unprejudiced 



A 



B 



FIG. 149. Comparison of Tornaria larva with larval Echinoderms. 

 [After O. HAMANN.] Main ciliated bands in black, lesser systems cross- 

 lined. 



(A) Tornaria, ventral view. (B) Tornaria, dorsal view. (C) Auricularia, ven- 

 tral view. (D) Bipinnaria, ventral view. 



mind would not hesitate to classify with these latter. These 

 larvae are all of about the same size, all bilateral in structure, all 

 transparent and equipped with bands of cilia, and there is even 

 a close correspondence in the manner of disposal of these 

 bands. 



In the case of the echinoderms the universal occurrence of 

 such larvae is taken everywhere as a proof that they represent 

 an early stage in the history of the Class, and that the ancestors 

 of these radiate, crawling, or sessile, bottom forms were bi- 

 lateral and pelagic. Now it would be highly improbable that 



