378 OILMAN A. DREW. 



The anterior adductor muscle (figs. 25 and 36, aa.) is 

 formed somewhat earlier than the posterior adductor muscle, 

 and throughout life the former is larger than the latter (fig. 

 48). When the anterior adductor muscle is first formed (fig. 

 25, aa.), it occupies a position at the anterior end of the 

 dorsal space, very near the apical plate. Soon after the test 

 is shed, it becomes surrounded by tissue that is drawn up 

 around it (figs. 35 and 39, aa.). The posterior adductor 

 muscle is formed soon after the test is shed (fig. 39, ^:ia.). It 

 lies ventral to the intestine, and posterior to the visceral 

 ganglia, and from its first appearance is surrounded by other 

 tissue. 



In the adult the adductoi's are attached to the shell, with 

 their dorsal borders very near the ends of the rows of teeth. 

 The function of these muscles is simply to close the shell. 

 The contraction of the muscles, and the consequent closing 

 of the shell, compresses an elastic pad known as the cartilage, 

 that lies in the cartilage pit. As soon as the adductor 

 muscles relax, the expansion of this piece of cartilage opens 

 the shell. The epidermis is not thickened to form a pro- 

 minent external ligament. 



The foot is attached to the shell by three pairs of well- 

 developed foot muscles, and by a number of fibres that form 

 a more or less connected series on each side, ventral to the 

 genital organ and liver. Of the three pairs of foot muscles, 

 one is posterior and two are anterior. The posterior muscles 

 are inserted on the shell along the bases of the teeth, 

 anterior and dorsal to the posterior adductor muscle. They 

 extend anteriorly and ventrally along the sides of the foot, 

 and form the strong retractors of the foot. The two pairs of 

 anterior foot muscles are attached to the shell close together, 

 along the bases of the teeth, posterior and dorsal to the 

 anterior adductor muscle. In distribution, the anterior pair 

 of these muscles correspond to the two anterior pairs in 

 Yoldia. They spread out along the sides of the foot, and are 

 distributed to its posterior and ventral portions. The more 

 posterior of the two pairs of muscles passes between the pair 



