Till. TUNICA //■>•. 



137 



of eel-graas. Od placing a good-sized V.s- 

 cidian in a vessel of fresh sea-water it will 

 be found to consisl of a 3emi-transparenl or 

 quite opaque test with tun openings, one 

 lower than the other, as seen in Mohj 

 which looks, when the two orifices are pro- 

 truded, like a double-necked bottle I Fig. ill). 

 The anterior or higher orifice or mouth is 

 for the passage of currents <»!' water into the 

 respiratory sic; ami the p or, usually 



lower, excurrenl orifice for the passage <>ut- 

 wards of fecal matter. The tesl or outer 

 skin is either delicate ami semi-transparent, 

 or it may he quite tough ami opaque. 



The Tunicates may in general terms be 

 characterized as having a usually rounded or 



-like body, which is sometimes barrel- 

 shaped, bilateral, with a dorsal and ventral 

 symmetry, protected by a transparent or 

 dense test, containing cellulose, lined within 

 l>\ a tunic surrounding the body-cavity. 

 There are two openings in the test, one oral, 

 the other "atrial;" the mouth leads into a 

 capacious pharyngeal respiratory sac, open- 

 ing posteriorly by an oesophagus into the 

 stomach, which i- provided with a liver; 

 the intestine is flexed, and ends near the 

 esophagus. The uervous system is bilat- 

 eral, forming a double ganglionated chain 

 in Appendicularia, but is reduced in the 

 typical A.scidians to a single ganglion, sit- 

 uated within the tunic between the two open- 

 ings. There is a t nhiilar heart, opening al 

 each end, and its beatings are often revere 

 the blood flowing in and out at either end. 



A singular group of Tunicate- is repre- 

 sented by Salpa, which is a pelagic fornj, 



rp*r 





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