//. rUNICATA: TETIIJOIDEA. 



50'J 



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 ])rebranchial 

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

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

 ous blind vesicles filled with excreta. 



From the eggs are hatched small swimming tadpole-like larvm 

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



aa h i 



Fig. .547. — Development of an Ascidian, {After Kupffer and Kowalevsky.) ?, larva, 

 just hatched ;--:•, cross-section through the tail of a slightly younger larva: ..;, much 

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

 larva just before attachment, {i*, Phalln^i'i mentula ; 3, k. fh. nutmniillata.) au^ 

 eye; c, notochord; d, tunic; d, digestive tract; d\ its nutritive, c/", its respira- 

 tory division; e. atrial vesicle; ek, ectoderm ; en, entoderm; A, brain ; /, oral in- 

 vagination ; ?'i, muscles of tail; 7(, neural tube ; 7(e, 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 larvre into the sessile ascid- 

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

 by means of three ventral anterior papillre; (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; 



