PHYSIOLOGY OF THE INVERTEBRATA. 379 



naked-eyed MednsK exhibit peculiar movements on being 

 alarmed ; but I am not sure whether these are, as is most 

 probable, purely involuntary, or performed with the view of 

 affording protection to the more vital parts of the animal. 

 Possibly the object may be to decrease the buoyancy of the 

 nectocalyx, and so escape from the source of injury by sinking 

 in the water. At any rate, these peculiar movements consist 

 of a sudden folding together of the entire nectocalyx, con- 

 sequent on an abnormally strong contraction of the swimming- 

 muscles ; and this contraction, besides being of unusual 

 strength, is also, of unusual duration. Thus the last idea of 

 this movement will perhaps be gained by regarding it as a 

 sort of spasm. The time during which this spasmodic con- 

 traction lasts is pretty uniform in different individuals of the 

 same species ; but it varies in different species from three to 

 six seconds or more. In all cases the disappearance of the 

 spasm is comparatively gradual, the nectocalyx re-expanding 

 in a slow and graceful manner, instead of with the rapid 

 motion characteristic of ordinary swimming. These move- 

 ments only occur when the animal is being injured, or 

 threatened with injury." (Romanes.) 



Romanes has shown that the lithocysts are the exclusive 

 seats of spontaneity, so far as the so-called " primary move- 

 ments " are concerned ; and he has failed to detect the slightest 

 evidence of spontaneity on the part of the contractile zones, as 

 asserted by Dr. Eimer. 



The tentacula of the Mediism are prehensile organs capable 

 of seizing upon and destroying animals of far more complicated 

 structure than themselves. 



The Echinodekmata. 



In these animals the muscular system is well developed ; 

 its fibres are flat and without transverse stri^. 



The natural movements of the Ecldnodermata have been 



