W. K. BROOKS ON THE GENUS SALPA. 183 



brings about adaptive changes in other parts. One of these is the differ- 

 entiation of a stomach for the retention and digestion of the food, in the 

 direct course of the gut. As long as the food was mixed with great 

 quantities of water, digestion and assimilation probably went on simul- 

 taneously in all parts of the post-pharyngeal gut, but as the water found 

 another exit and the food thus became more compact and solid, the 

 stomach of appendicularia became established and thus divided the gut 

 into an oesophageal, a gastric, and an intestinal region. 



Our knowledge of the primitive vertebrates seems to me to be too 

 scanty to show whether this differentiation occurred before or after the 

 tunicates diverged from the ancestors of the vertebrates. We are now 

 concerned with the history of the tunicata line alone, and the fact that 

 the differentiation now exists in all tunicates shows that it was brought 

 about very early in their history. 



Another most important change in the relations of the gut also took 

 place very early in their history. The intestinal portion became bent 

 upon the enlarged pharynx so as to form a cj with the intestinal bar of 

 the cj ventral to the pharyngeal portion, and with the anus on the ventral 

 middle line under the pharynx. Herdman represents the primitive con- 

 dition of the digestive tract of tunicates as a d, with the intestine and 

 anus dorsal instead of ventral (page 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 tunicates. 



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

 more efficient as an organ of locomotion, while the fa3ces 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 retained, 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 loco- 

 motor. 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 



