SALPA IN DELATION TO EVOLUTION OF LIFE. 193 



ment, the 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 function is solved." 



Although it seems as if the delicate walls of the gut of a bur- 

 rowing 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 embryology of every chordata 

 animal which is known to exist evidence of descent from an ances- 

 tor 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 discovery 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 

 endostyle to its homologies, we are told by Dohrn that it is equiva- 

 lent 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 

 5 



