144 THE STORY OF ANIMAL LIFE 



and purposes a kind of" worm; and, like many of 

 the higher worms, it has a larva with bands of 

 cilia. This larva, which is better represented in 

 some species than in others, was originally 

 described under the name of Tornaeria. It is 

 considered to resemble, in some degree, the larva 

 of Echinoderms; on this hint, some zoologists 

 have sought to establish a connection between 

 Vertebrates and Echinoderms, and have been 

 able to find other points of comparison besides 

 the one named. It remains to be seen whether 

 this suggestion will lead to further results. It 

 may be added that the larva of Balanoglossus has 

 also been compared with that of Phoronis (p. 122), 

 thus assuming a relationship with the Polyzoa, 

 and through them with the Brachiopoda. It ap- 

 pears, therefore, that the subject of the possible 

 relationships of the Vertebrata is one of the 

 greatest complexity. The last named theory, 

 however, has been adversely criticised by very 

 high authority. 



We have not, however, explained yet what is 

 the claim of Bala'noglossus to be grouped with 

 the Chordata. This consists in the fact that a 

 certain part associated with the interior of the 

 proboscis has been identified, from its structure, 

 mode of origin, and relations with the nerves, as 

 a notochord. Balanoglossus also agrees with the 

 true vertebrates in possessing successive pairs of 

 perforating gills (see p. 142), which are especially 

 noticeable in the young animal. The presence 

 of this feature is important, in view of the fact 

 that some authorities have sought to throw doubt 

 on the genuineness of the notochord of Balano- 

 glossus. 



Balanoglossus is not without relations, some 



