46 INVERTEBRATE ZOOLOGY. 



Class II. Caducichordata Tail and notochord present only in larval 



life. 



Sub-class I. AsciDiACEA Body sack-like. Pharyngeal wall form- 

 ing a sort of lattice- work. Excurrent 

 and incnrrent orifices generally approxi- 

 mated. 



Order 1. Ascidiae Sessile forms, either solitary {^nonasci- 



diae), or colonial {synascidiae) and ar- 

 ranged in generally stellate groups, 

 known as coenobia. 

 ■^ Bolteuia. Botryllus. 



Order 3. Pyrosomiae Free-swimming, transparent colonies of 



cylindrical or cone-shaped forms. Incur- 

 rent openings upon the exterior, cloacal 

 openings in the interior. -^ Pyrosoma. 



Sub-class II. Thaliacea Free-swimming, transparent cask-shaped 



forms. Pharynx with two rows of small 

 gill-slits, or a single pair of large gill- 

 slits. Oval and cloacal openings at oj^po- 

 site poles — often with alternation of gen- 

 erations. 



Order 1 . Doliolidae . . Two rows of gill-slits. Muscle-bands in 



the form of closed rings. Mantle thin — 

 generative cj^cle including one sexual and 

 two asexual generations. -^ Doliolum. 



Order 2. Salpidae A single pair of gill-slits. Muscle-bands 



not as complete rings. Mantle thick. 

 Alternation of generations simple, soli- 

 tary asexual individuals alternating with 

 a chain-like sei-ies of sexual forms. 

 ■jf Salpa. 



Cladus III. Cephalochorda. 



This group includes but two closely allied genera. The anatomical details 

 are given in the definition of the Cladus -q. v. -j^ Amphioxus. 



