PROTOCHORDATA 585 



the Enteropneusta 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 Enteropneusta, 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 

 Vertebrata, backbone. A true tail is found only in the Chordata. 

 In them it is a very important organ, used primarily in locomotion 

 and maintaining position, though it may become an organ of pre- 

 hension or a weapon. 



SuBPHYLUM ENTEROPNEUSTA (HEMICHORDA) 



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

 restricted 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 Balanoglossida, burrowing worms 

 of the genus Balanoglossus and related, slightly different genera, and 

 the Pterobranchia, the remarkable little organisms Cephalodiscus and 

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

 houses which they secrete for themselves by their proboscis. 



The body of Balanoglossus (Fig. 438) 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 un- 

 divided, the collar and trunk segments each in two lateral halves. (The 

 trunk cavities send forward into the collar a pair of " perihaemal " pro- 

 longations at the sides of the dorsal blood vessel, mentioned below.) 

 A pair, left and right, or a single, \th, proboscis pore opens at the base 

 of the organ. The mouth opens on the ventral side, between the over- 

 lap of the collar and the proboscis stalk. A pair of collar pores open 

 backward from the collar cavities into the first gill pouch (see below). 

 Dorsolaterally on the first part of the trunk is on each side a row of 

 numerous small gill openings; these lead into deep pouches which 

 communicate with the pharynx each by a tall opening virtually 

 divided into two by a tongue bar, which hangs from the dorsal side 

 but does not quite join the ventral side as do the tongue bars in 

 Amphioxus. In the region of the gills, and a little way behind it, the 

 trunk is somewhat flattened above or has a pair of lateral ridges or 



