CHORD ATA 



435 



than many of tlie other members of Urochordata. Ciona 

 intestinalis, whicli may serve as a type of this group, is 

 abundant in the Mediterranean, and is found widely distributed 

 in temperate seas. It is a hyaline transparent creature, when 

 young presenting the appearance of opalescent glass, but as it 

 grows old it is apt to become overgrown with foreign qjrgan- 

 isms, etc., and presents an opaque appearance. It attains a 

 length of 5 or 6 inches. 



At one end the Cvma is attached to a rock or some other 

 foreign substance, at the other end the mouth is situated, 

 surrounded by eight lobed processes, this may be regarded 

 as the anterior end of the animal. About an inch behind 

 the mouth, and on that side of the animal which may be 

 regarded as the dorsal, another aperture surrounded by six 



tin. 



m. Mantle. 



e. Ectoderm. 



tc. Test cell. 

 tm. Matrix. 

 Uc. Bladder cell. 



Fig. 252. — Diagrammatic 

 section of part of mantle 

 and test of an Ascidian, 

 ■ showing the formation of 

 a vessel and the structure 

 of the test. After Herd- 

 man. 



s, s'. Blood sinus in mantle being drawn 



out into test, 

 mc. Mantle cells. 

 y. Septum of vessel. 



lobes is found ; this is the atrial pore, and through this the 

 water which has passed through the perforated pharynx and 

 purified the blood, the waste matter from the intestine, and the 

 generative cells are all discharged. 



The whole animal is enveloped in a hyaline timic or test, 

 which is a cuticular excretion of the ectoderm, and into which 

 ectodermal cells and blood-vessels wander. The test is turned 

 in for a short distance at both the oral and atrial openings ; at 



