40 



arose along with the Enteropneusta and the early Chordata 

 by a common root from the Vermes, close to the point of 

 origin of the primitive Mollusca, or possibly a little further 

 back. This branch is the largest and most important in the 

 table. It probably very soon broke up into two series of 

 ancestral forms ; (1) those which lead upwards to the Verte- 

 brata, and to which we shall return later on ; and (2), the 

 common ancestors of the Enteropneusta and Echinodermata. 

 This latter series is now represented by Balanoglossus, 

 which may be regarded as the termination of the primary 

 branch having as lateral off-shoots the Echinodermata on 

 the one hand, and the Proto-Chordata on the other. 



The relationship between the Echinodermata and the 

 Enteropneusta is shown by Tornaria, the larva of Balano- 

 glossus agreeing with the typical Echinoderm larva in all its 

 most important characteristics.* The ancestral forms of this 

 branch may be regarded as being derived from the Trocho- 

 sphere (or possibly from a rather more generalised form 

 which had not yet acquired all the special Trochosphere 

 characters), after separating from which, they must have 

 acquired such peculiar features as the longitudinal post-oral 

 band of cilia, and the derivation of a water-vascular system 

 from the archenteron. From such an ancestor Balanoglossus 

 has probably been derived very much as we see it developing 

 at the present day from the Tornaria stage, by the disappear- 

 ance of the longitudinal band of cilia, the conversion of the 

 praB-oral lobe into the proboscis, the elongation of the body 

 into a worm-like form, and other changes. 



The line along which the Echinodermata were evolved is 

 not so easy to trace. Probably their first ancestors (fig. 16) 

 differed from the ancestors of the Enteropneusta in that 

 portion of the coelom (diverticula from the archenteron), 

 which formed the water-vascular system being developed 

 * See Balfour, Comp. EmbryoL, vol. i, p. 485. 



