TTPE PROTOCnOBDATA. 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. 

 TTP£! PROTOOEORDATA. 



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



chord a small flngerlike 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. Cephalodis- 

 cus, Rhabdopleura. 



3. Order Enter opneusta. — Free forms not forming colonies ; not 

 secreting a test ; intestine straight ; collar region without 

 lophophorelike processes. Balanoglossus. 



II. Class Cephalochobda. — 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. Amphioxus. 

 III. Class Ukoohobda.— 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 separate from digestive tract. 



1. Order Larvacea. — Betaining the larval tail with notochord ; free 

 simple pelagic forms. Appendicularia. 



3. Order Ascidiacece. — Simple and sessile or colonial and occasion- 

 ally pelagic ; tail and notochord wanting in adult. 



1. Suborder Ascidiw 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, Molgula, Cynthia, BoUenia ; Co- 

 lonial, Clavellina, PeropJwra. 



2. Suborder AscidicB compositce. — Colonial forms, the various 



individuals embedded in a common test; if the various 

 individuals open into a common cloaca the colony is not 

 pelagic. Amarceoium, Bidemnum, Tridemnum, Bi- 

 stapUa, Botryllus. 



