662 THE INVERTEBRATA 



Hemichorda the dorsal side of the gut at the anterior end, over the 

 mouth, forms a skeletal outgrowth into the proboscis. This out- 

 growth has received the same name as the notochord, on the theory 

 that it represents the anterior portion of that structure. 



(5) With the exception of the Hemichorda, all the Chordata possess 

 the tail^ a postanal prolongation of the body in the direction of its 

 main axis, without viscera, but containing extensions of the other 

 principal organs — muscles, nerve cord, notochord, and, in the Verte- 

 brata, backbone. A true tail is found only in the Chordata, In them 

 it is a very important organ, used primarily in locomotion and main- 

 taining position, though it may become an organ of prehension or a 

 weapon. 



An interesting biochemical confirmation of the morphological find- 

 ing that these subphyla, including the Hemichorda, are a unity, lies 

 in the fact that all of them possess a phosphagen^ which is a compound 

 of creatin, whereas the phosphagen of non-chordate animals is a 

 compound of arginin. Moreover these two phosphagens have been 

 found together, on the one hand, in the lantern muscles of a sea 

 urchin, a member of that invertebrate phylum which shows most 

 affinity with the Chordata (p. 2); and on the other hand in Balano- 

 glossus, the chordate which shows most affinity to the Echinodermata. 



SuBPHYLUM HEMICHORDA^ 



Chordata without tail, atrium, or bony tissue; with notochord re- 

 stricted to the preoral region ; central nervous system partly or wholly 

 on the surface of the body ; and three primary segments of the coelom 

 retained in the adult in corresponding, externally visible regions of the 

 body, the foremost of which is preoral. 



This small group contains the Enteropneusta, burrowing worms of 

 the genus Balanoglossus and related, slightly diflferent genera, and 

 the Pterobranchia, the remarkable little organisms Cephalodiscus and 

 Rhabdopleura, which live at considerable depth in the sea, in tubular 

 houses which they secrete for themselves by their proboscis. 



The body oi Balanoglossus (Fig. 463) has a conical preoral lobe, the 

 proboscis, which behind, by a narrow stalk, joins the short, wide collar 

 region. This overhangs in front the stalk of the proboscis and behind 

 the beginning of the long trunk. Each of these regions contains one 

 of the three segments of the coelom, the proboscis segment undivided, 



^ A phosphagen is a labile compound of an aminoacid with phosphoric acid. 

 It is intimately associated with muscular contraction, being broken down 

 during activity and reconstituted during rest. This is a more immediate source 

 of energy than glycolysis. 



^ Some authors give the name Enteropneusta to the whole of this sub- 

 phylum. 



