CHAPTER III 



TUNICATA {continued) 



CLASSIFICATION : LAEVACEA APPENDICULAKIANS STKUCTUEE, ETC. 



ASCIDIACEA SIMPLE ASCIDIANS — SPECIFIC CHARACTERS 



COMPOUND A SCIDIANS GEMMATION MEROSOMATA HOLO- 



SOMATA PYEOSOMATIDAE THALIACEA DOLIOLIDAE • 



SALPIDAE GENERAL CONCLUSIONS PHYLOGENY. 



We now turn to the systematic classification of the group ; and 

 further details of structure or function, points of interest in the 

 life-history such as budding and the formation of colonies, the 

 habits and occurrence, and other peculiarities such as phos- 

 phorescence, will all be noted under the orders, sub-orders, families 

 and genera in which they occur. 



CLASS TUNICATA. 



The Tunicata or Urochordata are hermaphrodite marine 

 Chordate animals, which show in their development the essential 

 Vertebrate characters, but in which the notochord is restricted to the 

 posterior part of the body, and is in most cases present only during 

 the free-swimming larval stages. The adult animals are usually 

 sessile and degenerate, and may be either solitary or colonial, fixed or 

 free. The nervous system is, in the larva, of the elongated, tubular, 

 dorsal, Vertebrate type, but in most cases it degenerates in the 

 adult to form a small ganglion placed above the pharynx. The 

 body is completely covered with a thick cuticular test (" tunic ") 

 which contains a substance similar to cellulose. The alimentary 

 canal has a greatly enlarged respiratory pharynx or branchial sac, 

 which is perforated by two or many more or less modified gill- 

 slits opening into a peribranchial or atrial cavity, which communi- 

 cates with the exterior by a single dorsal exhalent aperture (rarely 



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