658 CompaTative Animal Physiology 



These highly speciahzed characteristics of excitation of nematocysts in 

 Anemonia are pecuharly adapted to the normal functional roles of these 

 effectors. The value of having the nematocysts withhold their discharge un- 

 til the instant of mechanical contact is readily seen in comparing the results 

 of stimulating discharge by a piece of cotton soaked in bile salts with that 

 by a similar piece soaked in an extract of normal food material. In the for- 

 mer case the nematocysts are induced to discharge before contact by the dif- 

 fusion outward of the bile salts, which are highly potent cnidoblast excitors, 

 and therefore before the discharge can result in any attachment of the dis- 

 charging filaments to the cotton fibers, as occurs in the case of normal food 

 substances. 



The cnidocils, when present, are probably concerned with the excitation 

 process. That they are non-essential, however, is indicated by the fact that 

 they are absent in many of the Anthozoans. It would appear that in their 

 absence the cnidoblast surface comprises the receptive area for the response. 



The mechanism of discharge of nematocysts is not well understood. Two 

 types of theories in particular have found favor among students in this field; 

 both theories have in common the postulation that an increased pressure 

 within the capsule forces out the filament. One theory considers the pres- 

 sure increase following stimulation to be the result of a passage of water 

 into the capsule. Some have believed the nematocyst operates as a simple 

 osmometer.**' ^ Others think the water inflow associated with a hydration 

 and swelling of colloids within the nematocyst in response to change in pH,*" 

 and a consequent swelling of colloidal material contained therein. A second 

 theory attributes the increase in pressure to a contraction of a fibrillar net- 

 work associated with the capsule. ^^ It is possible that mechanisms fitting 

 both theories are to be found among the numerous nematocyst types of the 

 various coelenterate species. The nematocysts of Metridiuin appear to dis- 

 charge as a result of inflow of water; those of Physalia appear to require the 

 activity of contractile fibrils of the nematoblast. 



Functional nematocysts have been found in the flatworm, Microstomum,^- 

 ^^' ^* and in the sea-slug, AeoUsS'- ^- ^' It has been conclusively demonstrated 

 that these nematocysts have been derived from the coelenterates on which 

 these animals feed. It is interesting that in the appropriation of coelenterate 

 nematocysts, these flatworms and molluscs selectively utilize only certain 

 types. For example, Microstovtum digests the volvents and utilizes the pene- 

 trants. The sea-slugs, feeding on Pennaria, utilize onlv the highly eff^ectivc 

 type known as the microbasic mastigophores, to the exclusion of other types. 

 The appropriated nematocysts are quite concentrated in the bodies of their 

 new carriers, and would appear to serve as effectixe defensive weapons. 



REFERENCES 



1. Calkins, G. N., Biology of the Protozoa (1933). Philadelphia, Lea & Febiger. 

 607 p. '^richocysts. 



2. Cantacuzene, J., C. R. Soc. Biol. Paris 92: 1133-1136 (1925). Nematocyst toxins. 



3. Cantacuzene, J., and Damboviceanu, A., C. R. Soc. Biol. Paris 117:136-138, 

 138-140(1934). Nematocyst toxins. 



4. Glaser, O. C, ;. Exper. Zool. 9:117-142 (1910). Nematocysts in Aeolis. 



5. Glaser, O. C, and Sparrow, C. M., }. Exper. Zool. 6:361-382 (1909). Discharge 

 of nematocysts. 



