Chordata. 149 



SUBBRANCH UROCHORDA. 



As an example of the urochordates we may take the 

 common ascidian. Such forms are sometimes called "sea 

 peaches" or "sea pears," indicating the size, shape, and 

 general appearance. They are attached by one end to 

 rocks or shells or even to a muddy bottom. There are two 

 holes, one at, and the other near, the free end. When the 

 living animal is disturbed, it ejects water from both' of 

 these holes, hence the more common name, " sea squirt." 

 The tough muscular external coat, or tunic, also gives the 

 name tunicata. They are all marine. 



Structure of an Ascidian. Inside the outer wall, or tunic, 

 is a lining, the pharynx, which hangs free below its attach- 

 ment near the larger opening, the mouth. The pharynx 

 is perforated by many small apertures through which water 

 is driven by cilia. From the space around the pharynx, 

 the peribranchial chamber, the water passes out through the 

 second, or exhalant, aperture. From the lower end of 

 the pharynx arises the gullet, which soon enlarges into the 

 stomach. A relatively short intestine empties into the 

 peribranchial chamber, where the outgoing water current 

 catches the refuse of digestion. There is a simple tubular 

 heart, which is unique in its action. After pumping the 

 blood in one direction for a few beats, it reverses its action 

 and sends the blood the other way. The nervous system 

 is very simple, consisting mainly of a ganglion between 

 the two apertures (see Fig. 97). 



Development of Ascidians. In the above account of the 

 structure of an ascidian there is no trace of relationship to 

 the other chordates, and so long as the structure of the 

 adult only was known, no one even guessed at its real 

 affinities. But the study of its development threw light on 



