432 THE EVOLUTION OF MAN. 



latticed gill-sac into the gill-cavity, and is removed from 

 there by the respiratory pore or excretory opening (a). A 

 ciliated groove traverses the ventral side of the gill-sac, the 

 same "hypo-branchial groove " which we found before in the 

 Amphioxus at the same place (Plate XI. Fig. li, y, 15, y). 

 The food of the Sea-squirt, like that of the Amphioxus, con- 

 sists of small organisms, Infusoria, Diatomacece, parts of 

 dismembered sea-weeds and sea-animals, etc. These pass 

 with the inhaled water into the gill-sac, and from the end of 

 this into the digestive part of the intestinal canal, first into 

 an extension answering to a stomach (Fig. 14, mg). The 

 small intestine connected with it usually forms a loop, 

 curving around toward the front, and opens in a vent (Fig. 

 153, a), not directly out, but fii-st into the gill-cavity ; from 

 here the excrement is removed, together with the inhaled 

 water and the sexual products, through the common ex- 

 cretory opening (a). The latter is sometimes called gill- 

 pore, or respiratory pore {jiorus hranchialis), sometimes the 

 cloacal opening (Plate XI. Fig. 149). In many Sea-squirts^ a 

 glandular mass, representing the liver, opens into the intes- 

 tine (Fig. 14, lb). In some, there is another gland near the 

 liver, which is supposed to be the kidney (Fig. 14, u). The 

 real body-cavity (coeloma), which is filled with blood and 

 surrounds the stomach, is very small in the Ascidian, as in 

 the Amphioxus, and equally in both cases is usually con- 

 fused with the gill-cavity, which is filled with water. 



In the mature Sea-squirt there is no trace of a noto- 

 chord, an inner bony axis. This adds interest to the fact, 

 that the young animal, as it emerges from the egg, has a 

 notochord (Plate X. Fig. 5, ch), above which lies a rudimen- 

 tary medullary tube (Fig. 5, m). In the mature Sea-squirt, 



