510 



CHORD AT A. 



(4) Two dorsal invaginations are formed, these grow together, en- 

 velop the pharyngeal region, and form the atrium and peribranchial 

 chamber. It is to be noted that this arises from the dorsal sur- 

 face and extends ventrally, while the peribranchial chamber of 

 Amphioxus arises by folds which grow ventrally over the pharynx. 

 Besides sexual reproduction many ascidians reproduce by bud- 

 ding. Where this occurs it results in the formation of colonies, a 

 matter of systematic importance. 



Sub Order I. MONASCIDI^E. Simple ascidians of considerable size ; 

 sometimes with transparent, sometimes with thick opaque tunic. The 

 CLAVELLINID.<E produce small colonies by basal budding, each individual 



FIG. 548. 



Fia. 549. 



FIG. 548. .4. Molguln manhattensis* ; B, Eugyra pillularis* (From Verrill.) 

 FIG. 549. Botryllus violaceus. (After Carpenter.) ^4, small colony of eighteen indi- 

 vidual groups; BI two individual groups somewhat enlarged. 



with its own test; Perophora* CYNTHIID^E, test leathery, oral and atrial 

 openings four-lobed; Cynthia* MOLGULID^E, oral opening, six-lobed, 

 atrial four-lobed. Molgula* Eugyra* 



Sub Order II. SYNASCIDL55. Compound ascidians consisting of 

 numerous small individuals imbedded in a common cellulose tunic and 

 forming considerable crusts on stones, plants, etc. Usually (fig. 549) the 

 individuals of a colony are divided into small groups, the oral openings 

 (6-20 in number) forming a rosette around a common central atrium. 

 Distaplia* Leptodinum* Polyclinum* Amaroucium* Botryllus.* 



Sub Order III. LUCLE. Free-swimming pelagic synascidians, having 

 the form of a hollow cylinder closed at one end. The animals imbedded in 

 the tunic vertically to the axis of the cylinder, the oral apertures on the 

 outside, the atrial in the central cavity. Pyrosoma, very phosphorescent, 

 tropical, some species four feet long. 



Order III. Thaliacea (Salpaeformes). 



These, tike the Luciae and Copelatae, are pelagic, and play an 

 important part in the plankton, either by the vast numbers of 

 small individuals or by the formation of colonies of considerable 

 size. In form a Salpa may be compared to a barrel formed out- 

 side of a cellulose tunic, lined internally with a muscular wall. 

 The muscles run circularly (fig. 550), are six or eight, not always 



