W. K. BROOKS ON THE GENUS SALPA. 187 



blood-spaces which were most nearly transverse to the current of water 

 from the mouth to the primary clefts, were the ones which were first 

 made definite by natural selection, and that" the arrangement of the gill- 

 slits was thus determined. 



We can only conjecture how this unknown ancestral swimming 

 organism first became fixed, but the discovery of its descendants on the 

 modern sea-floor is among the possibilities of future explanation. 



The sedentary habit undoubtedly came gradually, and at first it may 

 have been temporary, confined perhaps to the breeding season, when, 

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

 fragments of crinoids, or the shells of trilobites or brachiopods or mol- 

 luscs, 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 below 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 concen- 

 tration. The attitude of the animal upon the bottom undoubtedly 

 determined the dorsal instead of ventral location of the common aper- 

 ture and of the median atrium or cloaca. As each step in this process of 

 concentration must have been advantageous, its evolution by natural 

 selection is easily intelligible. The accumulation of faeces 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 acquired. 



The folding of the originally straight digestive tract of the primitive 

 chordata into a U with the anus and intestine ventral to the pharynx, 

 took place in the ancestral tunicates as an adaptation to locomotion, 

 but, as appendicularia shows, it incidentally brought the anus into the 



