//. TUNIC AT A: TETHTOIDEA. 



509 



is the ganglion in the dorsal wall between oral and atrial open- 

 ings. Below it (rarely above it) is a branched subneural gland 

 which, from its relations and its opening into the prebranchial 

 part of the alimentary tract, has been compared to the vertebrate 

 hypophysis. In many there exist special excretory organs, numer- 

 ous blind vesicles filled with excreta. 



From the eggs are hatched small swimming tadpole-like larvae 

 (fig. 547), resembling Appendicularia and, like it, consisting of 



FIG. 547. Development of an Ascidian. (After Kupffer and Kowalevsky.) 1, larva, 

 just hatched ; 2, cross-section through the tail of a slightly younger larva; 3, much 

 younger stage, formation of notochord and nervous system ; U, anterior end of a 

 larva just before attachment. (2, Phallusia mentula ; 8, A, Ph. mammillata.) cm, 

 eye; c, notochord ; cZ, tunic: d, digestive tract; d', its nutritive, d", its respira- 

 tory division; e, atrial vesicle; ek, ectoderm; en, entoderm; h, brain; t, oral in- 

 vagination ; ni, muscles of tail; n, neural tube ; ne, neurenteric canal ; o, otocyst. 



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 three parts are recognizable: in 

 front a vesicular brain with a simple eye and an otocyst 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 ascid- 

 ians four processes are important: (1) The larvae attach themselves 

 by means of three ventral anterior papillae ; (2) The tail is retracted 

 and, after preliminary fatty degeneration, is absorbed; (3) The 

 body becomes more or less spherical by development of the tunic; 



