MOLLUSI A 



243 



called the Acephala (Gr. a, without, and KecfxtX/j, head), as well 

 as several other names, include all the Mollusca with bivalve 

 shells. They are mostly marine, although some genera live in 

 fresh water. The body of the animal is generally more or less 

 completely inclosed in the shell, the two valves lying on the 

 right and the left sides respectively and joined on the dorsal 

 side of the body by a hinge. 



The shell generally shows ;i series of concentric markings 

 or ridges, which indicate successive stages of growth, and are 

 arranged concentrically about a point on the dorsal side, called 

 the umbo ; this is sometimes in the middle of the dorsal side, 

 but usually nearer the anterior end (Fig. 251). The shell is 



Fir,. 251. Anodonta cygnea, a fresh-water mussel. A, from the left side; B, from the pos- 

 terior end. d.p.a, dorsal pallial aperture; ex.spk, exhalant siphon; ft, foot; in.sph, inhalanl 

 siphon; /^.ligament; /;/, mantle; «w, umbo. (After Howes, from Parker and Haswell's 

 Manual.) 



sometimes further marked by radiating elevations, flutings, or 

 ribs, and in some species is provided with spines of considerable 

 length. The tropical shells are often very brilliantly colored. 



After the soft parts of the body have been removed, the inte- 

 rior of the shell, which usually has a pearly lining, exhibits some 

 characteristic markings (Fig. 252). Near the edge is a line run- 

 ning parallel with it, :he pallial line, which marks the attachment 

 of muscles in the mantle. In some shells this line does not fol- 

 low the edge at the posterior end of the shell, but makes a deep 

 indentation called the pallial sinus, where the muscles of a struc- 

 ture called the siphon are attached. Further, we find one or two 

 very smooth spots, which mark the attachment of the strong 



