CHAPTEE IX. 



TUNICATA OR UEOCHOEDATA. 



Animals more or less resem.bliiig in structure the 

 Sea Sqiiirt. 



Classes. — Laevacea, with persistent urochord. 



Thaliacea, or Free-swimming Ascidians. 

 AsciDiACEA, or Sea-squirts. 



The Tunicata, Ascidians, or Sea-squirfcs, as has been 

 already stated^ are sometimes classified with the Ver- 

 tebrates in a group called Chordata, and sometimes as a 

 separate group. The group receives its names (coated 

 animals, leather-bottle animals) from the fact that the 

 creatures have a tough cuticle or skin, which is com- 

 posed of a material called Tunicin, similar in com- 

 position to vegetable cellulose. The mouth opens into 

 a cavity perforated with small respiratory holes (gill- 

 holes) ; this cavity also leads to the intestine, and 

 therefore unites the respiratory with a share in. the 

 alimentary function. Hence it has been called the 

 Pharynx, because that is the name given to the 

 enlarged part of a fish's throat which bears the gills, 

 and to the cavity at the back of the mouth in man 

 and other animals, which both leads to the gullet, and 

 receives the openings from the nostrils [posterior 

 nares), and therefore unites alimentary and respiratory 

 functions. Surrounding the pharynx is an outer 



