W. K. BEOOES ON THE GENUS SALPA. 197 



I have tried to show that the structure and anatomical relations of 

 this system of organs in the tunicates are quite consistent with the view 

 that it was originally acquired for the purpose which it now serves, the 

 capture of food. 



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 conditions of primi- 

 tive pelagic life. 



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

 have great difficulty in believing that this simple mechanism is primi- 

 tive" (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 explana- 

 tion, 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 their food in some other way than by the forma- 

 tion of little vortices to sweep into their mouths everything within their 

 influence. The limbs of the swimming 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 which even the youngest larvaB 

 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 start- 

 ing-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 investment, the 



