SALPA IN RELATION TO EVOLUTION OF LIFE. 177 



128); but I shall show further on that the relations exhibited by 

 appendicularia are the primitive ones, from which we must derive 

 those which are exhibited by other tnnicates. 



By this change the tail was freed from the gut and was made 

 much more efficient as an organ of locomotion, while the faeces were 

 discharged from the anus into the current of water which set out 

 through the pharyngeal clefts. This latter feature may not have 

 been of any value so long as habits of active locomotion were re- 

 tained, but, as we shall see, it became very important at a later stage. 



The embryology of the ascidians shows that this arrangement of 

 the digestive tract was secondary ; that at one time it was straight, 

 extending into that region of the body which is now specialized in 

 appendicularia as a tail. The advantage to an active pelagic animal 

 of this change is obvious, since it permits the tail to become purely 

 locomotor. As each slight variation in this direction must have 

 given a slight increase in the freedom of movement, the shape of 

 the body of appendicularia is easily intelligible as the result of 

 natural selection, and while the change is complete in this, the most 

 primitive tunicate which we know, so that we can only conjecture 

 the transitional stages, the change itself is not a complicated one. 

 It presents little difficulty, although the resulting differentiation of 

 appendicularia into two regions or " segments," a body and a tail, 

 has been made the basis of much speculation. 



The great development of the pharynx and the reduction of the 

 tail to an organ of locomotion soon resulted in a pronounced change, 

 of the sort for which Dana long ago proposed the term cephalization. 



As the functions of the pharynx, and of its oral end in particular, 

 became more and more complicated and more and more exactly co- 

 ordinated, while those of the tail became simplified, the elongated 

 nervous system became differentiated in a corresponding way, and 

 its caudal portion became reduced to a caudal nerve, while its oral 

 extremity became evolved into a cerebral vesicle with sense-organs 

 and nerves in relation with the co-ordinated structures of the 

 pharynx. 



All the characteristics of appendicularia, except the structure of 



the heart and the structure and position of the reproductive organ, 



are thus seen to be intelligible as direct adaptations to a pelagic 



life ; for its distinctive features, as compared with other primitive 



4 



