262 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



taceans. The ancient Ostracoda, Leperditia, Aristo- 

 zoe, etc., have a straight hinge-line and subcircular 

 valves, which are united dorsally by a ligament. The 

 resulting form of the early condition of the bivalvular 

 shell in these two distinct classes is so strikingly simi- 

 lar, that it lends weight to our supposition that the form 

 is induced by the mechanical conditions of the case. 



" I think that the adductor muscles which close the 

 valves may also be demonstrated to be the necessary 

 consequence of the bivalvular condition. In the phyl- 



embryo stage (Fig. 59) the 

 valves are closed by a single 

 adductor muscle, which is the 

 simplest condition mechan- 

 ically possible to effect the 

 desired end. 1 This muscle 

 does not seem homologous 

 with any muscle in other 

 classes of mollusks, and is 

 probably developed from the 



Fig- 59- Ostrea edulis, embryo; 

 a ad, anterior adductor muscle; mantle muscles as a COnSC- 



/, mouth; a, anus; v, velum ; /<, q ue nce of the Conditions of 

 hinge of shell. (After Huxley.) 



the case. In support of this 



view, bivalvular crustaceans may again be cited. They 

 have an analogous adductor muscle, developed, of 

 course, on an entirely different line of descent, but under 

 closely similar mechanical conditions. At the completed 

 prodissoconch stage in all pelecypods, as far as known, 

 there are two adductor muscles, a second one having 

 developed in the posterior portion of the body. In 

 later life the anterior, the posterior, or both adductors 



iThis early adductor appears in the same position in many genera, and is 

 apparently characteristic of the class. It is the anterior of the two adductors 

 found in the later stages ; but it may be retained or lost in the adult. 



