133 



parts of the membranes obtained by maceration in acid, I could not 

 detect any portions of the supposed vessels projecting from their sur- 

 faces, or floating freely amid the tissues, in the same manner that we 

 may observe so commonly amid the membranes of the Corallidse 

 when treated in a similar manner. In a single instance, at one of the 

 torn edges of the membranes, I saw a very small piece of the gelati- 

 nous tube projecting from the fractured edge; and this fragment pre- 

 sented no clear and definite outline, such as we always find in disen- 

 gaged vessels, but the same irregular and gelatinous appearance that 

 is seen in these organs when in situ. The canals do not preserve the 

 same diameter throughout their courses, but occasionally expand to 

 twice their average dimensions, and again contract to their original 

 size. The diameters of these organs vary from - 50 V o to r £rv °f an 

 inch ; and one of the latter size which I measured expanded at one 

 part of its course to xvvn °f an inch, but this amount of dilatation 

 is not of frequent occurrence. The thickness of the gelatinous lining 

 in one of the large-sized canals was -z-nTrrrts of an inch, and in one of 

 the smaller ones 3 o£ 00 of an inch. 



From the whole of the characters presented by these curious canals, 

 I am strongly induced to believe them to be the Haversian canals of 

 the shell. Like the Haversian canals in the bones' of the higher 

 classes of animals, they anastomose with each other, and especially in 

 Ostrea, which we shall presently describe : their courses are equally 

 tortuous and irregular, and their parietes are both lined with a mu- 

 cous or gelatinous coat. It is true, the Haversian canals in the bones 

 of man are on an average T £-s- of an inch in diameter, while those of 

 shells are ra V g or 6 ere o °f an mcn '■> but their diminutive size in the 

 shell is not out of proportion when we consider the difference in the 

 size of the animals. There is another tissue apparent in the structure 

 of this shell, which draws still closer the alliance which exists be- 

 tween it and bone. If we examine the membranous remains by 

 transmitted light, with a linear power of 1000, we find embedded, in 

 the spaces intervening between the sections of the canals, an abun- 

 dance of the remains of exceedingly minute vessels or elongated ca- 

 vities, which measured ^is-sos °f an i ncn ^ n diameter, and which are 

 either the remains of minute inter-cellular vessels, or of cavities ana- 

 logous to the so-called corpuscular bodies of true bone ; and here 

 the discrepancy in the size of the organs is not so great, as one of 

 the smallest of the bone-corpuscles measured ^ 3 1 6 of an inch, while 

 one of the calcigerous tubes radiating from it was ^-j-f-s-g- °f an i ncn 

 in diameter. 



