626 R. J. HARVEY GIBSON ON THE 



surface of the dome, however, at the very apex, the inner layer becomes im- 

 pregnated to a slight extent by pigment granules. 



It is very doubtful whether the caries on the surface layer of the shell be a 

 natural condition ; the author is inclined to think that it is due to a parasitic 

 growth, and that the pigment is foreign matter, either belonging to the parasite 

 or introduced from the surrounding water into the canals formed by it. This 

 view is supported by the fact that in some places the shell is much more 

 decayed than elsewhere, and that some shells, though younger, are more carious 

 than others which are obviously of older growth. The shells examined were of 

 course not fresh, and so any possible inhabitant of these tubes could not be 

 detected. In the physiological section of this research, the subject will be 

 reinvestigated. Irrespective of the possible truth of the explanation above 

 suggested, it will be interesting to know what conditions are necessary for the 

 formation of this caries, and what are the agents and modus operandi employed. 



The general appearance of the shell has been described very frequently ; 

 indeed, in the older accounts of Patella the shell was the only part which was 

 described with anything like completeness. Carpenter, who, so far as the 

 writer is aware, alone has studied the shell microscopically, says that it consists 

 of three layers, inner and outer layers less compact, and a middle layer of 

 polygonal or prismatic cells. 



Dall (loc. cit.) quotes Williams as saying that " the lining membrane of 

 the branchiae is continuous, and therefore that it is highly improbable that 

 water penetrates into the circulatory system, as in some other molluscs." 

 With reference to that statement, it may be well to remember the nature of the 

 epithelium lining the branchiae, as indicated above (vid. " Respiratory system "). 



Reaumur mentions the existence of glands in the foot, and Born (loc. cit.) 

 says there are tubercles in the same organ ; and these authors affirm that from 

 these glands, or tubercles, there exudes a glue, by which the animal fixes itself 

 to the rocks. Adanson also speaks of suckers on the pedal surface ; and 

 Adams (Recent Mollusca, i. 465) asserts that the cavities which the limpet not 

 infrequently makes in some kinds of rock are made by spicula with which the 

 foot is provided. The writer has not been able to find the slightest indication 

 of glands, either beneath the surface or in tubercles ; nor has he been able to 

 see anything that could be mistaken for a sucker or a spicule of any kind 

 whatever, although he made a very thorough examination of the foot, not only 

 by superficial search, but also by many microscopical sections. 



6. Nervous System. — The nervous system is exceedingly complicated, but, 

 with moderate care, it may be dissected out in its entirety. 



There are altogether no less than eight pairs of ganglia; only three of 

 these, however, are of primary importance, viz., the cerebral, visceral, and 

 pedal (PL CLII. fig. 50). 



