346 OILMAN A. DREW. 



along the sides of the shell ventral to the genital mass and 

 liver. The more posterior of the two anterior pairs of foot 

 muscles passes between the pair just mentioned, and is dis- 

 tributed to the anterior and ventral portions of the foot. 



In the foot all of the muscles are closely bound together 

 by their own fibres and by interlacing fibres, so that many 

 movements occur that cannot be explained by direct pulls of 

 one or more muscles. It should constantly be borne in mind 

 that the attachments of the fibres are all along the sides of 

 the foot, and that many, if not most of the muscle-fibres pull 

 from one portion of the body-wall to another, without chang- 

 ing the relation of the body to the shell. Thus the flaps can 

 be spread apart after the shell has been removed. By com- 

 pressing the blood contained in the large spaces of the foot, 

 many movements, especially those connected with protruding 

 the foot, may be perforuaed. 



As in the case of Yoldia, the foot muscles are so large that 

 they are attached along a considerable portion of the dorsal 

 surface of the shell. I regard this as the result of the size 

 of specialised muscles, and do not agree with Pelseneer (13) 

 that it should be regarded as a primitive character. 



Byssal Gland. 



The byssal gland is formed as an invagination, just ante- 

 rior to the posterior projection of the foot, about the time 

 that the side flaps of the foot begin to form (fig. 39) . Although 

 there is but a single external opening, the gland at first con- 

 sists of right and left pouches that extend into the foot near 

 its posterior side. The cells forming the upper portion of 

 the gland soon become somewhat swollen, and do not stain 

 very well with haBmatoxylin. The lumen of the gland soon 

 shows traces of a secretion, but the secretion has never been 

 seen protruding from the duct. 



The gland soon enlarges to a remarkable extent, becomes 

 quite irregular, and the paired appearance disappears. At 

 this stage, which extends from about the time that the gill 

 acquires its third lobe (fig. 41) until about the time that it 



