III. 2 



DEVELOPMENT OF BALANOGLOSSUS 



57 



and ventral sections, exactly as in the production of the bipinnaria 



larva of a starfish. This arrangement differs essentially from the rings 



of cilia that pass round the body in the trochophore larva found in the 



annelids and other spirally cleaving forms. In later tornaria larvae 



there is, however, in addition to the longitudinal bands always a 



posterior ring of stout cilia (telotroch), and in large oceanic forms 



(which may reach 8 mm in length) the longitudinal band itself is 



prolonged into prominent tentacle-like loops (Fig. 30). The cilia of 



the posterior ring are purely locomotive, while those of the band set 



up feeding-currents converging to the 



mouth. As the larva becomes larger the 



ciliary surface needed for locomotion 



and feeding has to increase relatively 



faster than the increasing mass of the 



body, the latter following the cube but 



the former only the square of the linear 



dimensions. Accordingly the cilia of 



the locomotive ring become broadened 



and flame-like, while the convolutions 



of the longitudinal (feeding) band reach 



fantastic proportions. In some types, 



however (Saccoglossus), the pelagic 



phase is brief and the telotroch alone 



is formed. 



Finally the larva sinks, becomes con- 

 stricted into three parts, and undergoes metamorphosis into the worm- 

 like adult. This development is so like that of an echinoderm that it 

 would be necessary to consider the enteropneusts to be related to 

 that group even if no other clues existed. Such close similarity in 

 the fundamentals of development cannot be due to chance. 



These animals thus provide a very remarkable and sure demonstra- 

 tion that the chordates are related to the echinoderms and similar 

 groups. The general arrangement of the nervous system as a sub- 

 epithelial plexus, as well as the whole course of the development, show 

 the affinity with the invertebrate groups, whereas the hollow dorsal 

 nerve-cord and the tongue-barred gill-slits are by themselves sufficient 

 to show affinity with the chordates, this affinity being also perhaps 

 suggested by other features, such as the 'notochord'. As we have seen 

 already, affinities are not to be determined by single 'characters' but 

 by the general pattern of organization of animals and especiallv that of 

 their development. The organization of the enteropneusts is certainly 



Fig. 



30. Older tornaria larva seen 

 from ventral surface. 



Letters as Fig. 29; coel. proboscis 

 coelom. (After Stiasny.) 



