CHORD ATA. 191 



two to many gill clefts. There is usually a tad- 

 pole-shaped motile larva which possesses a tubu- 

 lar dorsal central nervous system and a noto- 

 chord restricted to the caudal region. 



Order 1. Larvacea. 



Small pelagic tunicates swimming throughout 

 life by means of a tail. With a persistent noto- 

 chord and a single pair of gill slits. (Appendic- 

 ularia, Oikopleura.) 



Order 2. Ascidiacea. 



Mostly fixed, solitary or colonial tunicates, which 

 in the adult are never provided with a tail and 

 have no trace of a notochord. The test is well 

 developed, the pharynx large and perforated by 

 many gill slits. In most ascidians the sexually 

 produced embryo develops into a tailed larva; 

 many ascidians reproduce by budding to form 

 colonies. (Ciona, Molgula, Cynthia, Perophora, 

 Botryllus, Amarcecium, Leptoclinum.) 



Order 3. Thaliacea. 



Pelagic tunicates which swim by forming cur- 

 rents in the water. The adult is never provided 

 with a tail or a notochord. The pharynx has 

 two or more gill slits. Alternation of genera- 

 tions occurs, and may be complicated by poly- 

 morphism. (Salpa, Doliolum.) 

 Sub-phylum 3. Cephalochorda. 



The notochord extends the entire length of the 

 body including the head. The body is meta- 

 merically segmented. (Amphioxus.) 

 Sub-phylum 4. Vertebrata. 



A brain is developed as an enlargement of the 

 anterior end of the central nervous system; the 

 notochord extends no further forward than the 

 middle of the brain, and a vertebral column and 

 cranium are present. (Cyclostomes, fishes, 

 amphibians, reptiles, birds, mammals.) 



Conklin: Organization and Cell Lineage of the Ascidian Egg. Jour. Acad. 



Nat. Sci., Philadelphia, 2d Ser., 13, 1905. 

 : Does Half of an Ascidian Egg Give Rise to a Whole Larva? Arch. 



f. Entwicklungsm. d. Org., 21, 1906. 

 Metcalf: Notes on the Morphology of the Tunicata. Zool. Jahr., 13, 



1900. 



