PROTOCHORDATA 669 



SuBPHYLUM TUNIC AT A {UROCHORDA) 



Chordata without coelom, segmentation, or bony tissue ; with a dorsal 

 atrium in the adult; notochord restricted to the tail, which is present 

 in the larval organization only; the central nervous system removed 

 from the surface of the body and in the adult degenerate ; and a test, 

 usually largely composed of a substance (tunicin) related to cellulose. 



In the adult form, the members of this group are extraordinarily 

 unlike the rest of the phylum. They have lost all the characteristic 

 features of chordate animals except the gill clefts, and are rather 

 shapeless objects which lead a sluggish existence by means of an 

 organization of a low grade. Most of them are sessile, and there is no 

 doubt that this habit has established the peculiarities of the group. 



We shall describe the organization and life of the Tunicata by 

 giving an account of a typical example, Ciona intestinalis of the British 

 coasts, one of the simple "ascidians". The adult of this animal 

 (Fig. 468) is a subcylindrical sac, which reaches a height of several 

 inches, sometimes nearly a foot, seated by the blind posterior end 

 upon some solid object on the bottom, and at the other bearing two 

 openings, a terminal mouth or branchial opening and an atrial opening, 

 seated on a tubular projection a little way below the mouth. This 

 projection, which marks the dorsal side of the animal, is known as 

 the atrial siphon. Beyond its origin the body narrows as the oral 

 siphon towards the mouth. The latter is surrounded by eight small 

 lobes with red pigment spots between them. The atrial opening has 

 six lobes. Both apertures can be narrowed and virtually closed. When 

 the animal is in water and has them open a current may be seen to 

 set in at the mouth and out at the atrial opening. By sudden con- 

 tractions of the body water may be forced out of both of them. 



The body is covered by a tough, translucent test, remarkable for 

 being composed largely of tunicin, a substance closely related to 

 cellulose, and therefore not to be expected in an animal. The test is 

 a cuticular secretion of the ectoderm, but contains cells of mesodermal 

 origin which have wandered into it, and ramifying tubes in which 

 blood circulates, which enter it at a point near the base of the sac. 

 Below the test lies the true body wall or mantle, which contains 

 numerous longitudinal and transverse strands of muscle by which the 

 shape of the body can be altered, and sphincters around the openings, 

 where test and mantle are tucked in for a short distance. " 



The alimentary canal (Fig. 469) begins as a tube, the stomodaeum 

 or buccal cavity, lined by the inturned test. This leads to the very 

 large pharynx, 3. circlet of tentacles standing at the junction. A short 

 prebranchial zone of the pharynx lies between the tentacle ring and 

 xht peripharyngeal band — a couple of ciliated ridges, which run round 



