TYPE PROTOCHORDATA. 639 



probably been derived from composite forms, Pyrosoma showing certain 

 affinities in its budding to Salpa, the fact of some individuals being 

 solitary not necessarily indicating a primitive character, since in the com- 

 posite forms no organic union exists between the various individuals of the 

 colony when they have reached maturity, so that the so-called colonies are 

 rather aggregations than colonies, and the forms more properly termed 

 social than colonial or even composite. 



SUBKINGDOM METAZOA. 

 TYPE PROTOCHORDATA. 



I. Class HEMICHORDA. Body divided into three distinct regions ; noto- 

 chord a small fingerlike diverticulum projecting forwards 

 from anterior portion of digestive tract, with which it 

 retains connection. 



1. Order Pterobranchia. -- Sessile colonial forms, secreting a 



"house"; intestine bent upon itself; collar region with 

 lophophorelike, tentacle-bearing processes. Cephaloclis- 

 cus, RJiabdopleura. 



2. Order Enteropneusta. Free forms not forming colonies ; not 



secreting a test ; intestine straight ; collar region without 

 lophophorelike processes. Balanoglossus. 



II. Class CEPHALOCHORDA. Free forms, not pelagic ; not secreting a test ; 

 body not divided into three distinct regions, with numer- 

 ous metameres ; notochord completely separated from 

 digestive tract and traversing the entire length of the 

 body. Ampkioxus. 



III. Class UROCHORDA. Sessile or free pelagic forms; secreting a test; 

 body in adult not divided into three distinct regions 

 and showing no indications of metamerism ; notochord 

 usually wanting in adult, present in tail-region of larvae 

 and entirely sepai-ate from digestive tract. 



1. Order Larvacea. Retaining the larval tail with notochord ; free 



simple pelagic forms. Appendicularia. 



2. Order Ascidiacece. Simple and sessile or colonial and occasion- 



ally pelagic ; tail and notochord wanting in adult. 



1. Suborder Aacidice simplices. Simple sessile forms or else 



budding from stolons and forming somewhat straggling 

 . colonies, the various individuals not enclosed in a common 



test. Simple forms, Jlolyula, Cynthia, Boltenia ; Co- 

 lonial, Clavellina, Perophora. 



2. Suborder Ascidice composite. Colonial forms, the various 



individuals embedded in a common test; if the various 

 individuals open into a common cloaca the colony is not 

 pelagic. Ainarcecium, Didemnum, Tridemnum, Di- 

 staplia, Botryllus. 



