SALPA IN EEL ATI ON TO EVOLUTION OF LIFE. 181 



when, loaded down with eggs, the animal may have learned to rest 

 upon the fragments of crinoids, or the shells of trilobites or brachio- 

 pods or molluscs, to avoid clogging its delicate ciliated and vascular 

 pharynx with sediment. At the point where the heavy anterior 

 end of the tadpole-shaped body rested, the ectoderm cells, just be- 

 low the mouth, probably became modified for the excretion of an 

 adhesive cement. 



The sedentary habit, which must have resulted in a still greater 

 economy of energy and a corresponding increase of size, undoubtedly 

 became more and more firmly established, and the changes which 

 followed and resulted in the evolution of the ascidian type are easily 

 intelligible as adaptations to a fixed home, although we have little 

 to show the sequence of their acquisition. 



So long as the animal led a free life the fate of the deoxidized 

 water after it left the gill-slits had no meaning, but with the fixed 

 habit came the need for driving it away as far as possible, and the 

 external apertures of the perithoracic chambers became small, 

 moved towards each other, and finally united to give to the 

 exhaled current the strength of concentration. The attitude of 

 the animal upon the bottom undoubtedly determined the dorsal 

 instead of ventral location of the common aperture and of the 

 median atrium or cloaca. As each step in this process of concen- 

 tration must have been advantageous, its evolution by natural 

 selection is easily intelligible. The accumulation of fseces from 

 the intestine, around a fixed animal, is so unsanitary that the anus 

 has disappeared in many sedentary metazoa, while in others, such 

 as the crinoids and the lamellibranchs for example, secondary 

 adaptations for sweeping away the refuse matter have been ac- 

 quired. 



The folding of the originally straight digestive tract of the 

 primitive chordata into a U with the anus and intestine ventral to 

 the phai'nyx, took place in the ancestral tunicates as an adaptation 

 to locomotion, but, as appendicularia shows, it incidentally brought 

 the anus into the region of the pharyngeal clefts. As the seden- 

 tary 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 



