I4O W. C. CURTIS. 



Here again death ensues unless the next stimulus in the series is 



o 



forthcoming, viz., the contact with the digestive juices of the 

 "sand shark's " stomach. When this stimulus is furnished the 

 tetrabothrian surviving the wreck of its teleost host develops into 

 the final adult condition. 



The foregoing is stated in sufficiently general terms to be ap- 

 plicable, mutatis mutandis, to any Cestode having an extra-uterine 

 development and whether or not the life history outlined for 

 Crossobothrium is the correct one does not affect the general 

 conception of Cestode life history which I am attempting to 

 portray. 



In stating the above I have spoken of the reaction of the embryo 

 of a given stage to its particular stimulus just as I spoke of the 

 reaction of the oosperm to the stimulus which initiates the whole 

 cycle. If we ask ourselves what is the essential nature of the re- 

 action of the oosperm to the delayed developmental stimulus we 

 must designate it as primarily a reaction manifest in so many cell 

 divisions and subsequent differentiations. In what does the re- 

 sult produced by any one of the stimuli, which, if the embryo 

 runs its whole course, become applied to each of the successive 

 stages, differ from the result produced by the stimulus which acts 

 upon the oosperm ? Cannot the result in each instance be for- 

 mulated in the same way, viz., that the stimulus causes cell 

 divisions and subsequent differentiation ? And should we not 

 speak of all of them as " developmental stimuli " ? 



The exact nature of the stimulus at each stage is a thing which 

 in spite of many technical difficulties would be open to investiga- 

 tion and something which we may hope eventually to understand, 

 but however diverse the stimuli might be we should still have, 

 as above stated, changes of the same nature resulting from each 

 successive stimulus. The stimulus which is in the first instance 

 something in connection with the sea-water is probably in the 

 other cases a change in nourishment incident to the change 

 of hosts, but in every case the result of the stimulus may be 

 stated in the same general terms, viz., cell division and subse- 

 quent differentiation which ends in a condition of stable equilibrium 

 in which the animal finally perishes unless the next stimulus is 

 forthcoming. 



