W. K. BROOKS ON THE GENUS SALPA. 195 



Minute pelagic animals, with soft bodies bathed on all sides by pure 

 water, do not need special organs of excretion or respiration, and it is not 

 at all probable that the pharyngeal clefts were originally respiratory ; but 

 it is easy to understand how the channels through which the water 

 flowed became converted into gill-slits, in accordance with the law of 

 change of function, as the descendants of the primitive tunicates grew 

 larger and became sedentary, and thus came to need respiratory organs. 



It may be argued that the thing to be explained is not the existence 

 of gill-slits, but their serial reduplication or metamerism. It may be 

 held that the metameric repetition of the gill-slits of ascidians forces us 

 to regard the ascidian pharynx as the primitive form, from which that 

 of appendicularia has been produced by " degeneration." As we are told, 

 however, by no less an authority than Dohrn (Studien, IX, p. 417, and 

 VIII, p. 61) that the great number of gill-slits in the ascidians is due to 

 secondary multiplication, "nachtragliche Vermehrung," this considera- 

 tion need not detain us. 



If the logical conditions of sound morphological philosophy admit the 

 possibility of "nachtragliche Vermehrung," and permit us to believe that 

 the twenty or thirty pairs of gill-slits which are found in ascidians are 

 to be traced back to the eight pairs which the primitive fishes are said 

 to have possessed, the same logic will surely permit us to believe, on 

 sufficient evidence, that they have arisen not from eight but from a 

 single pair like those of appendicularia. 



All the vertebrates have a peculiar organ known as the thyroid 

 gland, and while it holds no prominent place in our general conception 

 of a vertebrate, this gland is actually one of their most constant and 

 characteristic organs. 



In all the jawed vertebrates, from the sharks up to man, its typical 

 structure is adhered to so closely as to prove that the gland as it exists in 

 man is an organ of vast antiquity. In all these animals it is a ductless 

 gland, situated far back in the throat, behind the hyoid skeleton ; but at 

 an early stage in its ontogeny it is a part of the endodermal epithelium 

 of the pharynx, and it arises on the middle line just within the mouth. 



Its function in the jawed vertebrates is problematical, but these two 

 features in its ontogeny seem to show that far back in the remote past, 

 before it had assumed its characteristic form, it had another function 

 which stood in some direct relation to the mouth. 



The tunicate endostyle is a conspicuous organ which attracts the eye 

 of all observers, but its true structure was first demonstrated by Pol, 



