PHYLUM MOLLUSC A 187 



Members of the class Acephala show extreme range of size. 

 Many species of the finger-nail shells of the genus Pisidium are 

 under 2 mm. in length, while the giant Tridacna from the Indian 

 and Pacific oceans reaches a length of more than a meter. 



Structure of the Shell. — The shell is covered externally by a 

 thin, organic cuticula. The main bulk of the shell is composed 

 of calcium carbonate. In some species, the innermost layer of 

 the shell is composed of thin layers arranged parallel to the sur- 

 face. These lamellae are minute enough to diffract the light and 

 thereby produce an iridescence. The nacre, or mother-of-pearl, 

 as this layer is termed, occurs in many fresh-water mussels and 

 is especially conspicuous in the pearl oyster (Meleagrina). One 

 type of pearls is formed by the deposition of nacre about foreign 

 objects which have been introduced between the shell and the 

 mantle. Many of the Acephala have a non-iridescent, porcela- 

 neous lining of the shells. A short distance from the margin of 

 the shell the mantle is joined to it by a line of muscle fibers called 

 the mantle muscle. The line on the shell formed by the attach- 

 ment of this muscle is termed the pallial line. 



Adductor Muscles. — In many of the Acephala there are large 

 adductor muscles, one attached near the anterior and the other 

 near the posterior end of the shell. In the common sea mussels 

 (Mytilus) the anterior adductor is greatly reduced so that closure 

 of the shell depends chiefly upon an enlarged posterior adductor. 

 In the oysters and pectens there are two adductors in the larva 

 of which the anterior later entirely atrophies, leaving a single 

 large posterior adductor near the middle of the shell. 



The Siphons. — When the shell is nearly closed, the margins 

 of the two mantle lobes are pressed together tightly except in the 

 posterior region. Here the mantle lobes remain lightly separated 

 to form two openings termed the siphons. Of these, the ventral 

 or inhalant siphon is for the inflow of fresh water into the mantle 

 chamber and over the gills, while the dorsal or exhalant siphon 

 discharges water from the mantle cavity and carries the feces 

 along with the water. The margins of the mantle adjacent to 

 the siphons are, in some instances, fused together, thus leaving the 

 siphons as permanent openings. The mantle in the region of the 

 siphons frequently elongates and produces a siphon tube which 

 projects beyond the shell. Both siphons may be united in a 

 single tube or there may be two tubes entirely or only partially 

 separated. 



