ADDUCTORS. 
345 
deeply cleft and arises to the right of the median line but posterior 
to the origin of the tentacular retractors. 
Many differences in detail chai'actei’ize the muscular arrangements 
of each group, Testacella in certain of its species developing a paired 
series of lateral muscles to the lingual sheath, which augment the 
contractile power of the terminal pharyngeal retractor, whose attach- 
ment has been transferred to the lingual sheath in correlation with 
its great development. 
In the Pelecypoda the muscular system is strikingly symmetrical 
in accordance with their general organization, and consists chiefly of 
Adductors and Retractors, with other less important muscles having 
protractor or other functions. 
The Adductors {ad, to; duco, I lead) are the very thick and power- 
ful muscular pillars which connect together and close the valves of 
the shell of the Pelecypoda ; they are considered to originate as modifi- 
cations of the pallial musculature, and each muscle has its opposite 
ends attached to con-esponding areas of the concave internal surfaces 
of the right and left valves, their places of attachment being shown 
by the more or less perceptible scars or impressions left thereon, 
which vary in size and distinctness according to the age of the animal. 
The adductors have long been utilized for facilitating the classifica- 
tion of the Pelecypoda, and although their employment for this 
purpose is not now so fashionable as that of certain other organs of 
the body, yet if reasonable allowance be made for aberrant or 
specialized forms, and probably less allowance than required by other 
modes of arrangement, they remain as valuable and reliable for the 
purpose as any, furnishing also data for the phylogenetic history of 
the group and marshalling together the most nearly allied species. 
According to the number and relative development of the adductors 
the Pelecypoda are divided into three groups : the Isomya or Dimya, 
the Heteromya, and Monomya. 
The Isomya (hVos, equal ; pi’s, muscle) is represented by Anodonta 
and other of our species, and possesses paired adductors aiqn'oxi- 
mately equal in size; the Heteromya (erepos, different; pi's, muscle) to 
which gToup our Dreissensia belongs has two unequal muscles, the 
anterior one being in process of degeneration and often conspicu- 
ously smaller than the more centrally placed posterior one and 
tending to occupy a more and more anterior and less functionally 
important position in proportion to the enlargement and centraliza- 
