ABT. 11 ANATOMY OF FRESH-WATER MUSSELS REARDON 3 



portion of the anterior adductor muscle. This muscle passes down- 

 ward to the anterior ventral portion of the foot with its greatest 

 development in the anterior portion. 



The posterior retractor muscle lies on the anterior ventral portion 

 of the posterior adductor muscle. At this point it is a compact 

 cylinder of fibers which after a short distance spreads diagonally- 

 through the body and foot and supplies the thick musculature of the 

 foot. These muscles are antagonistic to the protractor pedis muscle 

 which lies just at the base of the anterior adductor muscle and pre- 

 sents, with the mantle intact, a crescent-shaped form. With the 

 mantle removed, it is seen to be fan-shaped, spreading outward over 

 a large portion of the body and foot. By its play the foot is capable 

 of great expansion. The two small muscles with their attachments 

 in the beak cavities serve to ret'ain there the dorsal portion of the 

 body. The pallial line muscles are inserted along the edge of the 

 mantle and form a delicate connection between the mantle and shell. 



The muscular system of the remaining species is the same as for 

 L. ochraceus. 



LABIAL PALPS 



Plate 1, fig. 7 



The labial palps surround the mouth and consist of two pairs of 

 thin contractile flaps which are subtriangular in shape with an inner 

 and outer palp on each side of the body. Their edges are attached 

 dorsally and are free ventrally. The outer edge of the outer palp is 

 attached to the mantle, with the inner edges of each palp united, and 

 the outer edge of the inner palp attached to the body mass. Ante- 

 riorly the palps are attenuated and at their confluence directly under 

 the anterior adductor muscle give contour to the oral orifice with the 

 attenuated portion of the outer palp forming the upper lip and that 

 of the inner palp forming the lower lip. 



The exterior faces of the palps are smooth. The inner faces are 

 strongly furrowed transverselj^ for about three-quarters of the dis- 

 tance, where the transverse furrows terminate abruptly and longitu- 

 dinal furrows begin and continue to the mouth where they curve 

 inward and pass into the gullet. These ridges bear cilia which sweep 

 along the food brought into the mantle cavity at the inhalent siphons. 

 The cilia on the transverse ridges check undesirable substances and 

 send them back into the mantle cavity, while the cilia of the longi- 

 tudinal furrows sweep the desirable food into the mouth. 



GILL3 AND KEPRODUCTIVK SYSTEM 



Plate 1, figs. 6, 7, and 9 



The noticeably semicircular form is the only peculiarity of the 

 gills of L. ochraceus. The marsupium occupies the hinder portion 

 of the outer gills. 



The ^lochidia are bookless. 



