460 THE EVOLUTION OF MAN. 



of Mantle Animals (Tunicata) and of Vertebrates. The 

 notochord of the Appendicularia is a long cylindrical cord 

 (Fig. 162, c), which serves to connect the muscles which 

 move the flat, rudder-like tail. 



Among the various retrogressions which are undergone 

 by the Ascidian larva after it has attached itself, the 

 degeneration of one of the most important parts of the 

 body, the medullary tube, is, next to the loss of the noto- 

 chord, of peculiar interest. While in the Amphioxus the 

 medulla steadily develops, that of the Ascidian larva soon 

 shrinks to the proportions of a small, insignificant nerve 

 ganglion, which lies over the mouth-opening, above the 

 gill-body, and which represents the exceedingly low mental 

 endowments of this animal (Plate XI. Fig. 14, m). This 

 insignificant remnant of the medullary tube seems to retain 

 no likeness to the medulla of Vertebrates, although it 

 originated from the same rudiment as the medulla of the 

 Amphioxus. The sense-organs, which had developed in the 

 anterior end of the nerve-tube, are also lost ; in the full- 

 grown Ascidian there is no trace of them. On the other 

 hand, the intestinal canal now develops into a very 

 capacious organ. This soon breaks up into two separate 

 parts a wide anterior gill-intestine for respiration, and a 

 narrow posterior stomach-intestine for digestion. In the 

 former the gill-openings form in exactly the same way as 

 in the Amphioxus. At first the number of gill-openings is 

 very small ; it afterwards, however, increases considerably, 

 and gives rise to a large, lattice-like perforated gill-body. 

 The " hypobranchial groove " originates in the central line 

 of the ventral side of this gill-body. The wide gill-cavity, 

 which surrounds the gill-body, also develops in the Ascidian 



