194 THE BIOLOGY OF HYDRA : 1961 



studies of many different coelenteiates so that all of the types may 

 be fully explored rather than merely the limited cnidom of hydra. 



I would like to briefly explore one other aspect of nematocysts, 

 namely their function and functioning. We have not yet arrived at 

 a point where any single explanation can be had as to how a nema- 

 tocyst discharges nor do we understand the meaning of diversity in 

 nematocysts. Diversity in some microscopic structures such as lepi- 

 dopteran scales and perhaps some sponge spicules seems not to 

 be adaptive. This is, they all perform the same function and as 

 long as a given size or distribution is maintained, variation in shape 

 and ornamentation apparently can occur without selective forces 

 coming into action. 



In nematocysts we do know that some of the diversity is adap- 

 tive. There is little doubt as to the role of stenoteles and desmonemes 

 in hydra and the recent work of Burnett, Lentz and Warren ( 1 ) 

 has shown that the desmonemes respond before the stenoteles, trap 

 the prey and hold it till the stenoteles discharge and kill it. Also, 

 it appears clear from the work of Ewer ( 6 ) that the atrichs discharge 

 against smooth surfaces and presumably are sticky, or are gluti- 

 nants. Ewer also showed that foodstuffs or extracts from food in- 

 hibited the atrichs while enhancing the discharge of stenoteles. Any- 

 one who has worked with nematocysts has soon discovered that not 

 all types respond to all stimuli, and some types like atrichs and spiro- 

 cysts are very difficult to discharge under most conditions. How- 

 ever, with all the work that has gone on we still can identify only 

 three functions for nematocysts as far as the biology of the animal 

 is concerned, namely adhering, entangling and penetrating, al- 

 though Ewer ( 6 ) has suggested that the holotrichs of hydra may be 

 purely defensive. The penetrating types are all assumed to deliver 

 toxins and poison to the prey or foe but this has not been proven. 

 We have no described or specific function for most nematocyst 

 types and in fact our knowledge is limited in that what is known 

 about function comes entirely from hydra. The work on the nema- 

 tocysts of other types of coelenterates has concerned itself with 

 biochemical problems, with studies of discharge mechanisms, the 

 toxins and the makeup of the capsule rather than the function of 

 the many varied types. Thus we are left with about twenty types of 

 nematocyst of which we known the function of three. It would ap- 



