34 lU V. L. Guikling's Observations on the Chitonida:. 



at leisure. If the existence of the operculum is doubtful, or the animal 

 has withdrawn itself from sight, a specimen may be fractured and suf- 

 fered to rot in spring water, when the putrid mass must be carefully 

 washed and examined in a watch-glass. By these means I have delected 

 the spurious operculum in species which I beheved possessed it, but in 

 which it could not, in the common way, be detected after the most pa- 

 tient examination. In Colomhella it is sometimes so minute as to require 

 a sharp eye, or even a magnifier, before it can be found. In such cases 

 it is indeed spurious, or only the rudiment of the organ, which may be 

 more perfectly developed in other species, or in kindred genera, which 

 from their economy require an ampler shield against the attacks of 

 enemies. 



We are apt, however, to make use of this word spurious without suffi- 

 cient consideration. We should recollect, when wondering at the small- 

 ness or weakness of the horny opercula of some Mollusca, that the spe- 

 cies which possess such either live under the sand, reside in safety on the 

 coasts, or quit the waters when they are not feeding, the shell being held 

 down close to the rocks by a dried mucous secretion, as in some Turbi- 

 nidcB, or by the mere adhesion of the foot, as in Purpura, &c. The 

 operculum, which in many cases would not close the expanded aperture, 

 is only brought into use in cases of great peril, when the hold of the 

 adhesive foot is loosened, the vessels are emptied of mucus, the various 

 secretions, or the poisonous or coloured fluidsby which the enemy is to be 

 driven back or baffled, and the animal retires into the narrower whorls, 

 for which alone the operculum is fitted. When the operculum is per- 

 fectly solid and testaceous, we may be sure that its possessor commonly 

 resides in places where it is subject to the sudden attacks of dangerous 

 pursuers. Here it will be of ample size, and capable of closing the 

 larger and exterior whorl. The structure and composition of this organ 

 indicates the habits of the inhabitant in so many cases, that its value in 

 generic characters is far greater than many are willing to allow. 



Before concluding these notes on the Chitonida, I cannot refrain from 

 again referring to the complex and wonderful organs of the mouth for com- 

 minuting the food. The Palato-oesophagal membrane, when the animal is 

 plunged into boiling water, is easily detached, and forms a beautiful and in- 

 teresting object for the microscope. The anterior termination is expanded 



