188 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



region of the pharyngeal clefts. As the sedentary habit became slowly 

 established the anus became shifted from the middle line into the 

 exhaled current from the left perithoracic chamber, and finally into the 

 margin of its aperture, so that, during the migration of the exhalent 

 openings, the U of the digestive tract 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 doliolum 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 salpae. In the Polyclinida3 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- 

 clinidas 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 sediment. 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 

 simplification 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. 



