45^ THE EVOLUTION OF MAN. 



ment yet occurs ; two small sense-organs make their appear- 

 ance in the foremost part of the medullary tube : of these 

 the one is, according to Kowalevsky, an eye, the other an 

 organ of hearing of the simplest structure. A heart also 

 develops on the ventral side of the animal, on the lower 

 wall of the intestine ; and this is of the same simple form, 

 and is situated in the same place as the heart in Man and 

 all other Vertebrates. In the lower muscle-wall of the 

 intestine a wart-like growth makes its appearance — a solid 

 spindle-shaped cord of cell, — the interior of which soon 

 becomes hollow : it begins to move by contracting in oppo- 

 site directions, now backwards, and then again forwards, as 

 in the full-grown Ascidian. In this way the blood-fluid, 

 collected in the hollow muscular pouch, is driven in alter- 

 nate directions into the blood-vessels, which develop at both 

 ends of this tubular heart. A main vessel traverses the 

 dorsal side of the intestine, another its ventral side ; the 

 former represents the aorta (Fig. 161, t) and the dorsal vessel 

 of Worms. The latter represents the intestinal vein (Fig. 

 161, v) and ventral vessel of Worms. 



When these organs are complete, the progressive Onto- 

 geny of the Ascidian is at an end, and retrogression now 

 commences. The freely-swimming Ascidian larva sinks to 

 the bottom of the sea, relinquishes its power of free loco- 

 motion, and becomes fixed. By means of that very part 

 of its body which was foremost in locomotion, it adheres 

 to stones, marine plants, shells, corals, and other objects at 

 the bottom of the sea. To secure it to these, several 

 excrescences are employed, usually three wart-like bodies, 

 which may be observed on the larva, even while it yet 

 swims. The tail, which is of no further use, is now lost 



