192 W. K. BROOKS. 



The simplest explanation of its origin is that which attributes it 

 to the preservation by natural selection of a long series of slight 

 changes, each of which improved the adaptation to the simple con- 

 ditions of primitive pelagic life. 



Dohrn disputes this position, and says that " many persons would 

 have great difficulty in believing that this simple, mechanism is 

 primitive " (Studien, VIII, p. 62). The future must show how 

 many of these persons there are, but I shall now lay before 

 them Dohrn's own explanation, that they make comparisons for 

 themselves. 



"We ask," he says (p. 62), "how the ancestors of the tunicates 

 obtained their food before the endostyle was formed. Obviously 

 they were free swimming animals, and therefore in the position 

 to seize their food by hunting. It is as certain that they needed 

 other contrivances than the ciliated furrows and the slime-gland, as 

 it is that the ancestors of the cirripeds sought food in some other 

 way than by the formation of little vortices to sweep into their 

 mouths everything within their influence. The limbs of the swim- 

 ming forefathers of the cirripeds were certainly different from 

 the cirri of modern barnacles ; even so were the ancestors of the 

 tunicates differently constructed from the modern ones, and before 

 the slime-gland and the ciliated grooves became the exclusive 

 means of nutrition, they must have been the accessory aids to some 

 more primitive mode of capturing food "... 



"Ammocoetes lives in the sand, into Avhich even the youngest 

 larvae bore. Although direct observations fail, it must be assumed 

 that the excretion of slime and the ciliation have some advantage 

 in the nutritive or respiratory functions of organisms which live in 

 the mud. May we not believe that, in spite of all the sifting 

 through the oral tentacles and the velum, the hard particles of sand 

 would be injurious to the delicate epithelium of the gut, if this 

 were not protected by a thick coating of slime ; that the ciliated 

 furrows are adapted for conveying this slime to the most exposed 

 parts, and that, in this function, they have their starting-point? 

 Once brought into existence, it is not remarkable to see these useful 

 structures further evolved until the whole mass of food is invested 

 with a slimy admixture to facilitate its passage through the gut. 

 It is not impossible that besides acting mechanically as an invest- 



