X§M$t 



CHAPTER XXXVII 



THE ORIGIN OF CHORD ATES, AND THEIR RADIATION AS AQUATIC 



ANIMALS 



From studies on all the groups of chordates and comparisons 

 between them, it is possible to arrive at an idea as to what the 

 original chordates must have been like. They were small, 

 bilaterally symmetrical, and no part of them was sufficiently 

 hard or resistant to be capable of preservation by fossilisation. 

 They were marine animals, as are their lowest representatives 

 at the present day. Some of these may now be considered, 

 for, although they are specialised often to the point of de- 

 generacy, and are of no use in the interpretation of the higher 

 chordates, they show some characters which assist in estima- 

 tions of the relations which the chordates bear to other animals. 

 Balanoglossus is a " worm-like " form, with the following 

 chordate features : gill-slits, ciliated grooves assisting in the 

 process of feeding, a dorsal nerve-cord which for a short 

 portion of its length is tubular, and a skeletal structure in 

 the anterior region of the body which is held to represent 

 a rudimentary notochord. From the latter possession it is 

 classed as a Hemichordate. The body is divided into three 

 regions, " proboscis," " collar," and " trunk," and is adapted to 

 its mode of life, which is burrowing in the sand. Its chordate 

 affinities are obvious from the characters just mentioned, but 

 other features ally it to a number of invertebrates, and espe- 

 cially the Echinodermata. The free-swimming larval form of 

 Balanoglossus, the Tornaria, is very similar indeed to the 

 larval forms of the Echinoderms, in general form, in the 

 arrangements of the bands of cilia which it carries, and in the 

 method of origin of the mesoderm. The latter arises as three 



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