THE TUNIOATES. 



137 



of eel-grass. On placing a good-sized As- 

 cidian in a vessel of fresh sea-water it will 

 be found to consist of a semi-transparent or 

 quite opaque test with two openings, one 

 lower than the other, as seen in Molgnla, 

 which looks, when the two orifices are pro- 

 truded, like a double-necked bottle (Fig. 177). 

 The anterior or higher orifice or mouth is 

 for the passage of currents of water into the 

 respiratory sac; and the posterior, usually 

 lower, excurrent orifice for the passage out- 

 wards of fecal matter. The test or outer 

 skin is either delicate and semi-transparent, 

 or it may be quite tough and ojjaque. 



The Tunicates may in general terms be 

 characterized as having a usually rounded or 

 sac-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 

 by 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 cesophagus into the 

 stomach, which is provided with a liver; 

 the intestine is flexed, and ends near the 

 cesophagus. The nervous system is bilat- 

 eral, forming a double ganglionated chain 

 in A23peudicularia, but is reduced in the 

 typical Ascidians to a single ganglion, sit- 

 uated within the tunic between the two open- 

 ings. There is a tubular heart, opening at 

 each end, and its beatings are often reversed, 

 the blood flowing in and out at either end. 



A singular group of Tunicates is rej^re- 

 sented by Salpa, which is a pelagic form. 



Fig. IT^S— Structure 

 of a compouna 

 Ascidian, Ama- 

 ro2cium. A, bran- 

 chial sac; rn, 

 stomach; K 

 intestine: c, 

 mouth; o', testis; 

 rr', effei'ent duct 

 of the testis; C\ 

 ovary; p\ egg in 

 the body-cavity ; 

 p" , eges in the 

 atriuni; n. anus; 

 o shows the site 

 of the Ijeart; I, 

 liver: e, openings 

 in walls of bran- 

 chial chamber. 



