ADELOCHORDA 285 



dorsal and one ventral. These are connected by a nerve ring 

 in the region of the collar. The dorsal nerve chain is tubular 

 in front of the nerve ring. 3. The dorsal wall of the digestive 

 tract is prolonged forward into the proboscis as a stiff tube of 

 cells which forms a supporting axis for the proboscis. Neither 

 of these features are found in any of the phyla so far described 

 but they are regarded as the homologues of the pharyngeal 

 gill slits, dorsal tubular nervous system, and notochord, re- 

 spectively, of the Vertebrates. The validity of the third 

 homology may be seriously questioned. 



619. PHYLUM X. Urochorda. The Urochorda are also called 

 Tunicata because of the tunic or test, a thick integumentary 

 structure formed by the mantle in many forms. This test 

 is remarkable because it contains cellulose, which is otherwise 

 found only in plants. The test is sometimes gelatinous but 

 is often extremely tough and resistant. Many Tunicata are 

 fixed but there are also free swimming forms. In the adult, 

 the animals are usually markedly degenerate. The body is 

 often sack-like in form. There is a large pharynx with gill 

 slits, a dorsal tubular nervous system and a notochord. The 

 food in minute particles is collected from the respiratory current 

 and directed to the oesophagus by the action of ciliated grooves 

 in the pharynx. The Tunicata are all marine. 



620. Class I. Copelata. This class comprises free swim- 

 ming forms in which the notochord persists in the adult. 

 The gill slits open directly to the exterior. The body is cask- 

 shaped and there is a flat tail. The mantle is readily cast off 

 and reformed. 



621. Class II. Tethyodea. The Ascidians or sea squirts 

 are for the most part fixed. The gill slits and vent open into 

 a chamber, "atrium," formed by folds of the integument. 

 The atrial opening is usually near the mouth. Both mouth 

 and atrial opening can be closed by muscular contraction. The 

 whole body can also be greatly contracted. From the pharyn- 



