A Simple Tunicate. 135 



Note that the only plane that will divide the animal 

 bilaterally passes through the long axis of each neck. Sub- 

 sequent study will show that this plane actually separates 

 the right from the left. The incurrent siphon arises from 

 the morphological anterior end, while the cloacal tube 

 extends from the posterior end. The shorter distance 

 between the two is along the mid-dorsal line, while the 

 greater distance marks the ventral line. Through the walls 

 a light-colored band will be observed, extending along the 

 anterior portion of the mid-ventral line. It is the endosfyle, 

 and, as subsequent dissections will show, lies along the floor 

 of the pharynx. 



The outer covering of the animal, " tunic ' : or " test" 

 should now be partially removed from the left side. This 

 covering will be found to be thick and tough : its outer sur- 

 face, except where the specimen was attached to others, 

 rough and dirty ; its inner surface smooth and easily sepa- 

 rated from the transparent underlying " mantle" 



The entire tunic may now be removed, and its only points 

 of firm attachment to the underlying mantle are found at 

 the margins of the incurrent and excurrent openings. 

 Through the transparent mantle the contained deeply 

 colored viscera are to be seen, while the mantle itself is 

 found to be provided with certain clear white bands of 

 muscle tissue. These muscle bands are especially abundant 

 on the tubes, where they are arranged in longitudinal bundles 

 and transverse fibres. How are they arranged on other 

 parts of the mantle? It is through the activity of these 

 muscles that the animals contract and forcibly eject the 

 contained sea-water. The transverse fibres form sphincters 

 around the mouth and at the base of each siphon, and move 

 the six oral and four anal flaps. 



