530 



HISTORY OF THE HUMAN BODY 



others, although active as larvae, soon settle to the bottom and 

 become sessile. Contrary to expectation, it is the latter which 

 in general have retained the more primitive characteristics, 

 while the free-swimming ones are often greatly modified. In 

 a typical sessile tunicate, like the one shown in Fig. 145, a, 

 the delicate and rather complex organism is shut within a 

 tough and often wrinkled external coat or tunic, in which 

 there appear but two definite structures, an incurrent and ex- 

 current orifice, through which the water is continually driven, 



a 



FIG. 145. Typical Tunicate. 



(a) External view, (b) Diagram of internal anatomy. 



IN, incurrent; and EX, excurrent canals; Ph, pharynx; INT, intestine; An t 

 anus; CL, cloacal chamber; Tun, tunic; Integ, integument; End, endostyle; GL, 

 ganglion; G, gonadic gland; CD, gonadic duct; GO, gonadic opening; H, heart. 



much as in sponges. The incurrent orifice, which is really 

 the mouth, leads into a capacious pharynx, and this continues 

 into an intestine which, as usual in sessile forms, becomes bent 

 upon itself and opens by an anal orifice not far from the phar- 

 ynx. The anus opens, not directly to the exterior, but into a 

 cloacal chamber, which is really outside of the animal but en- 

 closed within the outer tunic. This same chamber surrounds 

 the pharynx and receives the water taken into the latter 

 through a series of gill-slits, which, in larvse, are few in num- 



