5Q 



ORIGIN OF CHORDATES 



III. 2 



2. Subphylum Hemichordata (= Stomochordata) 



Class i. Enteropneusta 



Balanoglossus ; Glossobalanus ; Ptychodera; Saccoglossus 

 Class 2. Pterobranchia 



Cephalodiscus ; Rhabdopleura 



Fig. 21. Balanoglossus, removed from its tube and seen from the dorsal side. 

 abd. abdomen; atr. atrium; an. anus; c. collar; h.c. hepatic caeca; p. proboscis; ph. pharynx. 



(From van der Horst.) 



Fig. 22. Balanoglossus in its tube in the sand. (After Stiasny.) 



In the Hemichordata are placed animals of two types, the worm- 

 like Balanoglossus and its allies (Enteropneusta) and two sedentary 

 animals, Cephalodiscus and Rhabdopleura (Pterobranchia). The Entero- 

 pneusta are mostly burrowing animals (Figs. 21 and 22) varying in 

 different species from 2 cm to over 2 metres long. Several genera are 

 recognized (e.g. Balanoglossus, Saccoglossus, Ptychodera) and they 

 occur in all seas. Saccoglossus occurs around the British coast. The 

 body is soft, without rigid skeletal structures, and divided into 

 proboscis, collar, and trunk. The animals are very fragile and it is 

 difficult to collect specimens in which the hind part of the trunk 



