198 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



slime also acts chemically as an aid to digestion. If this is the case, it is 

 easy to understand how a peculiarity so useful to sedentary animals like 

 the ascidians, or to floating ones like the salpae, gradually assumed the 

 whole function of nutrition. Thus the problem of the change of func- 

 tion is solved." 



Although it seems as if the delicate walls of the gut of a burrowing 

 animal would be more effectively protected if slime were directly excreted 

 " upon the most exposed spots," than by this highly specialized system of 

 organs, we might yet believe that the system " has its starting-point " in 

 the habits of ammocoetes, if we did not find in the structure and embry- 

 ology of every chordata animal which is known to exist evidence of 

 descent from an ancestor in which it had attained, not a starting-point 

 merely, but its full development. 



The ontogenetic evidence that the vertebrate thyroid body was at 

 one time a pharyngeal gland opening just within the mouth, and the dis- 

 covery by Dohrn of rudimentary peripharyngeal grooves in the torpedo 

 embryo (Studien, etc., VIII, Plate H, Figs. 7f, 7g, 7h and 7i), seem to me 

 to be convincing proofs that the organs did not have their starting-point 

 in the habits of ammocoetes nor in any degenerated fish, but that they 

 arose in a lineal ancestor of the selachians and of the higher vertebrates, 

 which was also an ancestor of the tunicates and cyclostomes. 



Passing now from the biological relations of the system of the endo- 

 style to its homologies, we are told by Dohrn that it is equivalent to two 

 pairs of gill-slits ; that these gill-slits were present and functional in the 

 fish-like ancestors of the cyclostomes and tunicates, and that two of 

 them, the mandibular clefts, moved downwards and met on the ventral 

 middle line to form the thyroid gland or endostyle, while the endodermal 

 portions of the others, the spiracular clefts, lost their connection with the 

 exterior and became converted into the peripharyngeal grooves (Studien, 

 etc., VII and VIII). 



Homologies are expressions of genetic relationship, and Dohrn tells 

 us (p. 79) that they are valuable in phylogeny only as they furnish us 

 with the opportunity to pass from the consideration of the structure of 

 organs as they now exist, and of the functions of these organs at the 

 present time, to the consideration of conditions which have passed away ; 

 to the study of the history of the modifications which have come between 

 these structures and functions, and those which we must attribute to the 

 same organs at an earlier genealogical stage. 



I regard the structures which we find in the tunicates and in ammo- 



