II. TUNICATA: TETHYOIDEA 



445 



FIG. 499. dona intesti- 

 nalis, a bit of the wall of the 

 gill sac enlarged to show the 

 gill slits. 



arranged in longitudinal and transverse rows (fig. 499), through which 

 the water received from the mouth passes into the peribranchiai chamber, 

 thence to the atrium, and so to the external world. 



While the respiratory water thus passes out in a nearly direct course, 

 the food particles which it contains pass into the digestive tract. By 

 means of the peripharyngeal band just inside of the tentacles and sur- 

 rounded by mucus secreted by the endostyle the food is carried back to the 

 oesophagus (oe) at the base of the gill chamber, 

 and thence to the stomach (usually provided 

 with liver glands), and on to the intestine. The 

 anus is at the base of the special portion of the 

 peribranchiai chamber, which also receives the 

 genital ducts and hence is known as the cloaca 

 or atrium. 



In the body cavity, which is greatly reduced ", 

 in the species with compact bodies, occur the |j 

 digestive tract, the sexual organs, and the 

 heart; the latter (lie) frequently S-shaped, ex- 

 tends between the stomach and the endostyle. 

 Opposite to the endostyle is the ganglion (g) 

 in the dorsal wall between oral and atrial openings. Near it is a 

 branched subneural gland which has been compared to the vertebrate 

 hypophysis. In many there exist special excretory organs, numerous 

 blind vesicles filled with excreta. 



From the eggs are hatched small swimming tadpole-like lame (fig. 500), 

 resembling Appcndicularia and, like it, consisting of trunk and tail, in which 

 the chordate features are strongly marked. The digestive tract is confined to 

 the trunk; dorsal to it lies the tubular nervous system in which can be recognized 

 a vesicular brain with a simple eye and a statocyst imbedded in its walls; farther 

 back a narrower portion ('medulla oblongata'); lastly, a spinal cord extending 

 into the tail. In the axis of the tail is a notochord which extends forward a short 

 distance into the trunk between digestive tract and nervous system. In the 

 metamorphosis of the free larvae into the sessile ascidians four processes are 

 important: (i) The larvae attach themselves by means of three ventral anterior 

 papillae; (2) The tail is retracted and absorbed; (3) The body becomes more 

 or less spherical by development of the tunic; (4) Two dorsal imaginations 

 are formed, these envelop the pharyngeal region, fuse and form the atrium 

 and peribranchiai chamber. It is to be noted that this arises from the dorsal 

 surface and extends ventrally, while the peribranchiai chamber of Amphioxns 

 arises by folds which grow ventrally over the pharynx. Besides sexual reproduc- 

 tion many ascidians reproduce by budding. This results in the formation of 

 colonies, a matter of systematic importance. 



Sub Order I. MONASCIDLE. Simple ascidians of considerable size. 

 The CLAVELLINID^E produce small colonies by basal budding, each individual 

 with ils own test; Perophora* CYNTHIID.E, oral and atrial openings four-lobed; 



