258 Introduction to A n imal 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 (unifora), 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 (btfora). 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 (trifora). 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 linc y 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- 



