A TAXONOMIC STUDY ON PYROSOMA — METCALF AND HOPKINS. 263 



tion in the colony. This seems an even more remarkable instance 

 of the "feeling for form" permeating the wholo colony and affecting 

 even the nonliving, secreted material of which the colony is in large 

 part composed. Of course, this figurative manner of expressing the 

 point is false in its implications. The thing of moment is, that if the 

 test cells be not active in secreting the test substance or in controlling 

 its modeling, then there is some type of control which affects not only 

 the living cells of the ectodermal epithelia of the zodids, but the 

 secreted test substance as well, and the reaction has relation to the 

 form of the colony as a whole and not to its constituent zooids. 



There is a further point of some interest. On the hypothesis of the 

 direct response of the secreted test substance to controlling influences, 

 causing it to take the definite form and positions characteristic of the 

 four test "tentacles," we must say that these controlling influences 

 are in the nature of tropisms and the response is a non vital one. 

 Of course, it is wholly probable that these tropisms themselves are 

 established through the vital activities of the living constituents of 

 the colony, but the response of the secreted material, which causes it 

 to take definite form and position, is a nonvital response. If a 

 structure so definite in form and position as one of these test "tenta- 

 cles" is formed under the control of stimuli external to itself, by a 

 response evidently nonvital, it suggests how intimate and how inti- 

 mately controlling may bo the influence of such tropisms upon 

 living substance in this and other organisms. 



Whether the test cells be active (first h} 7 pothesis) or inactive 

 (second hypothesis) in the secretion of test material, P]irosoma pre- 

 sents peculiarly interesting conditions. The behavior of the isolated 

 test cells in Pyrosoma atlanticum and the formation of the test 

 sculpturings, especially the ridges and "tentaclos" of Pyrosoma 

 agassizi, are even more mysterious than the behavior of migrating 

 cells and tissues in the embryonic development of organisms, for 

 there is in the former loss of organic contact. We do not see that 

 we are as yet in possession of data that enable us to make any sug- 

 gestion as to the nature of tho control over those phenomena. The 

 problem, however, is presented in Pyrosoma in a most striking way 

 that challenges attention. 



GEOGRAPHICAL DISTRIBUTION. 



The distribution of oach species and subspecies of Pyrosoma has 

 boen stated in connection with our description of its structure, but it 

 is well to bring the data together. The accompanying table shows the 

 localities from which each form has been reported, and the authority 

 for the statement. Whero no authority is mentioned, the statement 

 is based upon tho collections of the United States Buroau of Fisheries 

 and the United States National Museum, discussed in this paper. 

 The question marks are explained each in a'foot note. 



