CHAPTER XVII. 



TUNICATA OR UROCHORDATA. 



The Tunicates, including the common Ascidians or sea- 

 squirts, are degenerate Vertebrates. The young forms have 

 a dorsal nervous system, an underlying notochord in the tail 

 region, gill-slits opening to the exterior, an eye developed 

 from the brain. But in most cases this promise is unfulfilled, 

 the larvae settle down and lose tail and notochord, nerve-cord 

 and eye, becoming quaintly deformed. 



In many adults the shape of the body is like that of a 

 bent wine-sack with two apertures. Through one the water 

 is drawn in, bearing food-particles with it ; through the other 

 it is swept out. Passing in by the mouth the water enters 

 a large respiratory pharynx whose walls are perforated by 

 numerous ciliated slits ; through these it passes into a sur- 

 rounding (atrial or peribranchial) chamber, and thence out 

 by the exhalent aperture. 



The cuticular tunic of the adult seems to contain a carbo- 

 hydrate nearly allied to, if not identical with cellulose, and 

 also some proteid substance. The nervous system is reduced 

 to a single ganglion, situated between the mouth and the 

 exhalent aperture, The heart is a simple ventral tube, which 

 drives the blood first in one direction then in the other. 

 There are no nephridia. The animals are hermaphrodite. 



Many sea-squirts (so-called from the force with which water 

 is sometimes driven from the exhalent aperture) are common 

 on or near the coasts of all seas. They are usually attached 

 to stones or ghells, or to the muddy bottom. Few indi- 

 viduals measure more than three or four inches in length, 

 and most of them less. They live on minute organisms 

 which are carried in by the water-currents. Many form buds 

 from their basal parts, and great clusters thus arise, e.g., 



