48 THE OPERCULUM. 



foot, its fibres are nearly vertical to the plane of the operculum, 

 which usually appears to be immediately superimposed upon 

 them : in Buccinum, however, Keferstein finds interposed a layer 

 of long cylindrical epithelial cells, with mostly distinct nuclei, 

 and long divided processes entering between the muscular fibres. 



The operculum, a cuticular development of these cells, is 

 composed, as may be seen in the corneous opercula of Murex, 

 Purpura, Triton, etc., of very thin superimposed layers. With 

 the microscope one may perceive in a thin section, the cylindrical 

 cells with their head attached to the lowermost layer ; or, on the 

 inner face, the small rounded pittings where they have been 

 attached. 



Homologies of the Operculum. Dr. J. E. Gray was, after 

 Adanson, the first investigator who considered the operculum 

 homologous with the right valve of the lamellibranchiates or 

 bivalve mollusks. He has shown that the operculum is developed 

 on the embryo long before it is hatched ; that it is placed on and 

 covers a particular part of the bod^^ called the lohus operculigerus 

 and which bears to it the same relation which the mantle does to 

 the shell, and that its growth occurs in the same manner ; that 

 this growth is made by the addition of new matter to the inner 

 surface and especially near the margin ; that it is attached to the 

 animal by means of one or more muscles, which, as in the bivalve 

 shell, pass from the larger valve or shell to the smaller one or 

 operculum ; that the operculum, as it increases in size, is 

 gradually moved on the end of its muscle — the many-whorled 

 operculum of the Trochi revolves as many times on the end of 

 the muscle as the many-whorled spiral shell turns on its imag- 

 inary axis ; that the operculum is often lined internally with a 

 shelly coat like a shell, and sometimes, like the Cowries, its outer 

 surface is covered also with a shelly deposit by a special devel- 

 opment of the opercular lobe. 



The principal difference between the operculum and the valve 

 or shell of the gastropods consists : — 



1. In the operculum having no cavity, its cone being depressed, 

 flat or even concave, or very much compressed, forming only a 

 spiral riband, as in the spiral operculum. But this absence of 

 a cavity is a difference only of degree, for the valves of some 

 gastropods, as Umbrella, Patella, etc., are much flattened ; the 

 flrst resembling the annular operculum of Ampullaria and 

 Paludina ; but the greatest resemblance is to be observed in the 

 small, flat valves of Grryph?ea, Exogyra, Chama,and other genera 

 of bivalve shells which are attached by one of their valves. 

 These valves are often quite as flat and destitute of any cavity 

 as the operculum of any gastropod ; and it is to be remarked 

 that these valves exactly resemble a spiral operculum in shape, 

 the remains of the ligament forming a spiral mark on the outer 



