190 W. K. BROOKS. 



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 multiplica- 

 tion, " nachtragliche Vermehrung," this consideration 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 con- 

 ception 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 tlie 

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

 Fol, who proved that it is a pharyngeal gland with excretory cells 

 to produce slime, and with ciliated cells to drive the slime out 

 through the long, narrow, slit-like duct iuto the pharynx. Fol 

 also showed its true relation to the ciliated peripharyngeal bands 

 and dorsal lamella, and proved by simple but conclusive experi- 

 ments that these organs are co-ordinated parts of a single system, 

 which has for its function the capture of the microscopic floating 

 food which enters the mouth with the water. 



