138 BULLETIN OF THE BUREAU OF FISHERIES. 



CHEMICAL COMPOSITION. 



The shell of the mussel consists of an organic base infiltrated with mineral salts, as 

 has been shown above in the description of its histological structure. 



The organic matrix is an albuminoid substance called conchiolin, the composition 

 of which, according to Wetzel (1900), is carbon 52.3 per cent, hydrogen 7.6 per cent, 

 nitrogen 16.4 per cent, and sulphur 0.65 per cent. It is readily obtained by mascerating 

 the shells in hydrochloric acid and boiling the residue in sodium hydroxide in which the 

 conchiolin remains undissolved. Treated with hot mineral acids it goes into solution. 

 Wetzel (1900) found that the substance gives Millon's reaction, and from the decomposi- 

 tion products formed in boiling sulphuric acid he obtained glycocoll, leucin, and an 

 abundance of tyrosine. He assigns this compound to a place between casein and egg 

 albumin. 



The inorganic constituents of the shell consist chiefly of calcium carbonate with 

 which are present small quantities of sulphates, oxides, or carbonates of magnesium, 

 iron, manganese, and silica. The following analysis by Mr. Adrian Thomas will serve 

 to show the various elements and compounds: 



Composition of the mussel shell. 



Calcium oxide 



Magnesium oxide 



Iron and manganese oxides 



Silica 



Carbonates 



Sulphates 



Organic matter 



Water 



Phosphates, chlorides, sulphides Trace. 



Traces of sodium and potassium which probably came from unremoved sea water 



were also detected. 



THE MANTLE. 



The mantle is a fold of integument which almost completely envelops the body. 

 It is composed of the two lobes which lie symmetrically placed on the right and left sides 

 of the body. They arise dorsally as an outgrowth of the body wall, cover the entire 

 inner surface of the shell, and terminate in a free ventral border which is firmly attached 

 to the edge of the shell by means of the pallial muscles. The free mantle edges unite 

 anteriorly near the posterior edge of the anterior adductor muscle. At the posterior 

 end of the shell they are joined together by a triangular-shaped band of deeply pigmented 

 integument, the branchial membrane (fig. 115, BrM , opp. p. 138). 



The exhalent syphonal opening (fig. 115, Exs, and fig. 116, Exs) lies just dorsal to 

 the branchial membrane and is surrounded by a tough ring of heavily pigmented tissue. 

 The mantle edges separate to pass on either side of this opening and converge forward to 

 the apex of the shell, where they unite and terminate. Between the syphonal opening 

 and their point of termination they are joined together by a continuation of the 

 branchial membrane. The space between the mantle lobes lying just below the exhalent 

 corresponds to the inhalent syphon of many lamellibranchs. 



The structure of the mantle lobes in young animals is quite simple. In cross section 

 they are thin and membranelike, consisting of an outer layer of simple epithelial cells, an 



