334 



A. T. MASTERMAN. 



Holocliorda (Vertebrata). 



Annelida and 

 Arthropoda. 



Eu-chorda. 



Chsetognatha. 

 Sipunculoidea 

 Poly zo 

 Bracliiopoda 



Cephalochorda. 



Urochorda. 



Archichorda.' 



(Actinotrocha.) 



(Archiccelomata.)s 

 Stage III. 



Hemichorda. 

 ^Diplochorda. 



Echinodermata. 



Stage II. (Pelagic nekton.) 



Stage I. (Pelagic plankton.) 



Diploblast. 



In the above tree the idea of the ancestry of the Chor- 

 data is a long line of pelagic organisms up to the stage 

 reached in Actinotrocha, at which stage two groups took 

 to degraded habits, the Hemichorda and the Diplochorda, 

 and thereby saved themselves from extinction. Balano- 

 glossus, with its burrowing habitat, shows a few remarkable 

 modifications and further differentiation in organs, and even a 

 commencing metameric segmentation, and is less degenerate 

 than the Diplochorda. In this group Cephalodiscus 

 has sought refuge in a deep-sea habitat and a protecting house ; 

 whilst Phoronis also resorts to a sedentary and, in most 

 cases, tubicolous habit. Hence the former retains a more 



' Differing in the main from Stage III by the evolution of paired chordoid 

 rudiments, rudimentary pharyngeal clefts, and a subneural gland. 



