ANATOMY AND LIFE HISTORY OF MOLLUSCA. 137 



the rods and spreads out on the inner surface, and the rods 

 themselves are reversed ; (5) there are special peculiarities in the 

 manner in which the nerves supply these organs, an illustration 

 of which can be seen at pi. vi., fig. 8, which is an eye found in 

 the substance of the shell of Patella tramoserica while being 

 ground down for a section. It need not appear improbable that 

 such eyes should become entombed and disused, for an instance is 

 met in the case of Trigonia margaritacea, where it will be shown 

 that large eyes like those found in the Chitonidse exist abundantly 

 on the interior lining of the shell. This instance will be referred 

 to subsequently. 



Mantle Eyes. — Organs similar to those described in the cases of 

 Pecten maximus, Lam., P.jacobeus, Lam., and P. opercularis, Lam., 

 have been observed in a few bivalves, and probably will be found 

 more numerously. It must not be supposed, however, that every 

 warty excresence on the mantle is necessarily an eye. Any one 

 who has visited a coral reef in the Southern Hemisphere cannot 

 fail to have been attracted by the beautiful appearance of the 

 clams. As the tide recedes the open valves display most 

 beautiful colours, especially in fringes and dots of the brightest 

 blue and green. These are species of Tridacna and Hippopus. 

 It may not be taken for granted that these dots are eyes, as 

 Brock has shown that they are warts and flask-like cysts, 

 probabty containing chlorophyll (see J. Brock, Uber die 

 sogenannten Augen von Tridacna, (fee.).* I do not, however, think 

 that this settles the question as to all the coloured warts which 

 are displayed upon the coral reefs, by species of Hippopus as well 

 as Tridacna. It should be a special enquiry with naturalists in 

 collecting in these localities as zo the true nature of these organs. 

 They are so large and the species are so abundant that they 

 should afford an easy, as well as interesting field of research. 



Associated with such mantle-warts are certain patches of pigment 

 cells which are found at regular intervals on the edge of the 

 mantle as already described. Under the microscope this pigment 

 has usually associated with it minute, highly refractive, spherical 

 cells, much larger than the refractive bodies associated with nerve 

 tissue, and in fact hardly mistakable for anything else than 

 lenses. Under the lens there is a cylindrical prolongation forming 

 a capsule not unlike the membrane forming the prismatic 

 structure in the substance of the shell. I shall mention hereafter 

 an instance of this in the case of a young specimen of Siphonaria 

 diemenensis where 80 to 90 of such visual or sense organs were 

 observed upon a portion of the mantle in front of the head. 



* Zeitschriit fur Wissenchaftliche Zoologie. Sechsundverzigster Band* 

 Leipzig, 1888, p. 280. A translation of this paper appeared in the Ann- 

 and Mag. Nat. Hist., June, 1888, p. 435. 



