258 Introductio7i to Animal Morphology. 



sists of conchiolin 4 per cent. ; calcium carbonate 94 per 

 cent. ; phosphoric acid, silica, iron oxide, &c., 2 per cent. 

 The optical properties of the calcium carbonate agree with 

 those of Arragonite. 



The mantle lobes may be free from each other 

 along the lower margin [iimfora)^ as in Ostrea and 

 Pecten, or they may be united at one spot, so as to 

 leave a small hinder excretory and a very wide ante- 

 rior slit {biford). In others the anterior opening is 

 subdivided by a second union, so that there is an an- 

 terior or foot opening, a middle or ingestory, and a 

 posterior or exhalant orifice [triford). The walls of 

 the latter pair of orifices may unite and be elongated 

 in the form of a double-barreled tube whose upper 

 canal is exhalant or excretory, and the lower is inha- 

 lant (or both may be used indiscriminately, Clark). 

 Sometimes the anterior or foot orifice is tubular, siphon- 

 like (Kellia). The mantle consists of epithelium on 

 both sides of a connective basement, having along its 

 free border muscular fibres and glands, and contain- 

 ing blood-vessels, or lacunae, and nerves — often ten- 

 tacles, &c. In Teredo the mantle contains cells like 

 the cellulose-holding elements of Phallusia. The line 

 of attachment of the muscular fibres of the mantle to 

 the shell is called the pallial line, and is roughly pa- 

 rallel to the lower edge of the shell, often interrupted 

 (Ostrea, Saxicava, Panopaea, &c.) When a siphon 

 exists, the hinder edge of the pallial line is indented 

 by a deep pallial sinus whose fundus points forwards, 

 and whose depth is proportional to the length of the 

 siphon (sometimes as in Cyclas a siphon may exist 

 with no pallial sinus). The wall of the siphon con- 

 tains circular fibres around each tube, and longitudi- 



