620 



INVERTEBRATE MORPHOLOGY. 



siphons, in whose walls circular muscles are developed to 

 serve as sphincters of the openings. The branchial siphon 

 opens posteriorly into the branchial region of the digestive 



Fig. 383.— Figuee of a Tunicatb, Seterotrema, removed from the Test 



(after Fiedler). 

 A = atrial pore. po = peripharyngeal ciliated band. 



an = auus. ' s = stomach. 



00 = cerebral ganglion. sn = subneural gland. 



en = endostyle. si = branchial stigma. 



ex = excretory organs, t = testis. 



/ = intestine. ■»(? = vas deferens. 



tract, the opening being known as the mouth, and usually 

 being surrounded by a number of tentacles (Fig. 283) which 

 arch over it. The atrial siphon, on the other hand, does not 

 open into the body proper but into a cavity, lined probably 



