176 W. K. BROOKS. 



any evidence of degeneracy, or in its habits any basis for the 

 assumption that it is degenerated. In most respects its structure 

 is like that of the hypothetical ancestor whose evolution we have 

 traced. It has an unsegmented notochord, and a capacious lumen 

 throughout the whole course of the digestive tract from mouth to 

 anus. This lumen is permanently distended and food is carried 

 through it by cilia. It has a blind diverticulum from the stomach, 

 and the greatly expanded pharynx opens laterally through two 

 ciliated pharyngeal clefts, through which the water escapes while 

 the food passes into the oesophagus. There is a ventral slime-gland 

 just inside the mouth, and its excretion is conveyed upwards around 

 the pharynx by the cilia of the peripharyngeal bauds, and is then 

 swept into the oesophagus with the entangled food. 



This increasing complexity and perfection of the pharynx is 

 accompanied by an increase in its size, so that in the primitive tuni- 

 cates it soon comes to be the most important and dominant organ 

 of the body, and brings about adaptive changes in other parts. One 

 of these is the differentiation 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 simultaneously 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 appen- 

 dicularia 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 differentation 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 diiferentiation 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 intesti- 

 nal bar of the d ventral to the pharyngeal portion, and with the 

 anus on the ventral middle line under the pharynx. Herdman 

 represents the primitive condition of the digestive tract of tunicates 

 as a cl> with the intestine and anus dorsal instead of ventral (page 



