232 



CHORDATA 



Order 1. Larvacea. 



Small pelagic tunicates swimming throughout 

 life by means of a tail. With a persistent 

 notochord and a single pair of gill slits. (Ap- 

 pendicularia, 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 ascidi- 

 ans the sexually produced embryo develops 

 into a tailed larva; many ascidians reproduce 

 by budding to form colonies. (Ciona, Mol- 

 gula, Styela, Perophora, Botryllus, Ama- 

 roucium, Leptoclinum.) 



Order 3. Thaliacea. 



Pelagic tunicates which swim by forming cur- 

 rents in the water. The adult is never pro- 

 vided with a tail or a notochord. The phar- 

 ynx has two or more gill slits. Alternation of 

 generations occurs, and may be complicated 

 by polymorphism. (Salpa, Doliolum.) 

 Class 3. Cephalochorda. 



The notochord extends the entire length of the 

 body including the head. The body is meta- 

 merically segmented. (Amphioxus.) 

 Subphylum 2. 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, 2nd 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. Jahrb., 13, 



1900. 



