93 



The shell of juveniles of M chinensis is lustrous yellow and trihedral. The 

 posterior part of the body has two transparent siphons and a well-developed 

 foot that enables the mollusk to move quickly over the seabed. 



Miyasaki (1933) has described the development of Mactra chinensis; 

 however, data are quite fragmentary. Larvae and juveniles of this species are 

 described by Hayashi and Terai (1964), including the hinge of the planktonic 

 larva and its external appearance. The early development of Mactra chinensis 

 has been described by Medvedeva and Malakhov (1983). 



SPISULA SACHALINENSIS (SCHRENK) 



(Mactridae) 



Veliconcha 



The larva attains this stage at a shell length of 300 )j.m. The shell is 

 equivalve, ovate, and obtuse at the posterior end; the anterior part is flattened. 

 The anterior end is larger than the posterior. The umbones are low, broad, and 

 shifted to the posterior end. The shoulders slope gently (Figure 6 1 ). The hinge 

 begins to form in the larva when the umbo appears. Initially, the hinge consists 

 of two primary plates with small denticles separated by a deep pit. Later, the 

 denticles on the primary plates disappear and the plates become massive. A 

 lobate tooth is present in the anterior part of the hinge of the left valve. The 



Figure 60: Veliconcha of Mactra chinensis. 

 right valve; B — hinges of right (1) and left (2) valves. 



