426 



THE AMERICAN NATURALIST 



[Vol. LIII 



irritated, or almost any other region (with difBculty in 

 the case of the tentacles), a greater intensity of stimnlus 

 is necessary than when applied on the back of the animal. 

 Mechanical stimulation is in general more effective than 

 chemical. Of the chemical excitants tried, only NaCl, 

 LiCl, methyl and ethyl alcohols were successful, whereas 

 such substances as pure amyl alcohol, numerous acids, 

 alkalies and salts, in concentrations corresponding to 

 those used in the case of the successful stimulating agents, 

 as well as a series of alkaloids, were without effect when 

 small volumes (a half or one c.c.) were applied to the 

 mantle. 



The designation of the mantle glands as repugnatorial 

 is justified by observations in nature. Isopods (Ligia) 

 and grapsoid crabs {Sesarma, and in some places, Pachy- 

 grapsus) abound in certain spots where the Onchidia take 

 their airing. By prolonged watching, unequivocal in- 

 stances have been noted in which these arthropods have 

 come into direct contact with Onchidia, and in every case 

 the glandular discharge took place, followed immediately 

 by the retreat of the crab or isopod; it is not implied that 

 the crabs wore endeavoring to eat the snails, but it seemed 

 ratlior that it was important for OiirJinlnim to avoid being 

 afvidciitally pnslied off tlie rock into the watrr; it does 

 not adhere to the rock witli any great tirmiiess, partic- 

 ularly under water, is easily dislodged, and if purposely 

 pushed over the edge of the stone on which it may be 

 creeping into the water, the animal finds it difficult if not 

 impossible to return to its own nest. 



Tests upon a number of animals (e. g., Fundulus from 

 the land-locked brackish ponds at Bermuda) which could 

 not by any possibility have ever come into contact with 

 the Onchiclium secretion, have invariably demonstrated 

 the powerfully repugnant character of gland contents. 



Finally, it i?> iaiportant to note that Onchidia of ivhat- 

 ever degree of pigmentation possess these repugnatorial 

 glands, and in an equivalent state of development. The 

 behavior of Oie differently pigmented individuals, with 



