182 W. K. BROOKS. 



became twisted into an 8 in such a way that, as Plate VIII, Fig. 

 2 shows, the intestine p passed on the left side of the oesophagus, 

 q, to open dorsally into the atrium, near the middle line, but a 

 little to the left. 



This arrangement of the digestive organs is very characteristic 

 of the tunicates, and the few exceptions are clearly due to later 

 changes. Thus in doliolura the atrium has moved backwards as 

 an adaptation to locomotion, and the anus has followed it until the 

 gut has become nearly straight. The intestine and anus of the 

 adult aggregated Salpa pinnata, Plate I, Fig. 1, are ventral; but 

 I have shown that in the young the intestine crosses to the left of 

 the oesophagus to open dorsally, as it does in the adults of all 

 ordinary salpse. In the Polyclinidae the loop of the intestine has 

 been elongated, with the elongation of the body, until the bend of 

 the 8 has been obliterated, and the presence of the characteristic 8 

 in more primitive ascidians such as clavelina shows that the Poly- 

 clinidse have been more recently modified. 



All sedentary animals which take their food by means of cilia 

 have their apertures raised in some way above the reach of sedi- 

 ment. In the crinoids this end is reached by a stalk ; in the 

 lamellibranchs it is attained either by siphons, or by the vertical 

 elongation of the shell as in the oyster; and the shifting of the area 

 of attachment of the ascidians from the oral end to the aboral end, 

 the elongation and approximation of the mouth and the atrial 

 aperture, the acquisition of oral and atrial sphincter muscles, the 

 degeneration and disappearance of the locomotor tail, and the sim- 

 plification of the nervous system, are such obvious adaptations to a 

 sedentary life that it is not necessary to discuss them. 



Section 3. — The Annelidian Hypothesis. 



I believe that the structure of the tunicates has been acquired as 

 an adaptation to the biological conditions which prevailed at the 

 surface of the primitive ocean, and that it has been evolved by the 

 gradual addition of successive complications on to the body of a 

 still more primitive and simple ancestor. This involves the total 

 rejection of the dogma that the vertebrates are modified annelids, 

 and that the tunicates are degenerated vertebrates. 



