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The leading vertebrate characteristics of the Timicata 

 are the uotochord, the dorsal nervous system, the ventral 

 heart, and the respiratory pharynx with gill clefts, but 

 these all disappear or undergo modification to such an 

 extent, during the metamorphosis, that the degenerate 

 adults would not, in most cases, be recognised as belong- 

 ing to the chordata were it not for our knowledge of the 

 life-history. 



The class Tunicata may be divided into three orders : — 



Order I. LARVACEA. 



This comprises the free-swimming, permanently-tailed, 

 larva like, mostly minute Appendicularians. A relatively 

 large test or "House" is formed with great rapidity as a 

 secretion from the surface of a special part of the ecto- 

 derm ; it is, however, merely a temporary structure, which 

 may be cast off and afterwards replaced by another 

 *' House." The branchial sac is simply an enlarged pharynx, 

 with two ventral ciliated openings (stigmata) leading to 

 the exterior. These open independently on the ventral 

 surface, and there is no separate peribranchial cavity. 

 The tail is a large locomotor appendage, in which there 

 is a skeletal axis, the urochord, comparable with the noto- 

 chord of Vertebrata. The nervous system consists of a 

 large anterior and dorsally-placed ganglion, and a long 

 nerve cord with smaller ganglia stretching backwards 

 from it over the alimentary canal to reach the tail, along 

 which it runs on the left side of the urochord. The 

 alimentary canal lies behind the branchial sac, and the 

 anus opens ventrally on the surface of the body in front 

 of the stigmata (or atriopores). The gonads are placed at 

 the posterior end of the body. Gemmation does not take 

 place, and alternation of generations and metamorphosis 

 do not occur in the life-history. 



