has not yet been studied from a biochemical 

 point of view. 



After the disappearance of their contained 

 glycogen the vesicular cells do not slirink or 

 collapse. A hypothesis was therefore advanced 

 (Semichon, 1932) that the glycogen granules are 

 supported by a framework of a special substance 

 which remains intact after the dissolution of 

 glycogen. It is claimed that this framework can 

 be revealed by staining with black anilin inks. 

 The evidence for the existence of such a special 

 substance is not, however, convincing. In cells 

 with a moderate content of glycogen the latter 

 can be seen in close contact with the protoplasmic 

 network typical for vesicular cells. Furthermore, 

 the walls of tlie vesicular cells are fairly rigid and 

 the cells retain their shape even wlieii they are 

 empty. The shrinkage of connective tissue fre- 

 quently caused by changes in osmotic pressure 

 when the salinity of the water smTounding the 

 oyster is suddenly increased is not associated 

 with the disappearance of glycogen. 



The fat globules in vesicular cells vary greatly 

 in size and number, usually forming distinct 

 vacuoles that are easily dislodged. The relation- 

 ship between the fat and glj^cogen content of the 

 oyster and the role of lipids in the physiology of 

 laniellibranchs have not been studied. 



Large oval cells containing a brown pigment are 

 scattered throughout the connective tissue of the 

 nuintle. The pigment is not soluble eitlier in 

 acids or fat solvents. Its chemical nature and 

 physiological significance are not known. 



\Vandering blood cells are commonly seen in 

 the mantle. They crawl between the connective 

 tissue cells, aggregate in the vicinity of blood 

 vessels and blood sinuses (fig. 79), and are grad- 

 ually discarded through the sui-face of the mantle. 

 As a rule, the oyster continually loses a certain 

 amount of blood by diapedesis or bleeding. Any 

 excess of heavy metals accumulated by lilood 

 cells (see p. 390) is also discarded by this normal 

 process. 



MUSCLES 



The radial muscles consist of large, regularly 

 spaced bands of fibers which extend almost the 

 entire width of the mantle from the line of its 

 fusion with the visceral mass and with the ad- 

 ductor muscle to the free margin. For a study of 

 the anatomy of the muscular system the con- 

 nective tissue in which the bands are firmly 

 enclosed should be macerated in 1 percent potas- 



sium hydroxide for about 24 hours. After being 

 washed in distilled water the loosened tissues are 

 removed with a small stiff brush and fine forceps. 



The radial muscle bands are composed of 

 large bundles of unstriated fibers which begin to 

 branch toward the distal edge of the mantle about 

 one-third of the distance from that edge. At 

 this level the muscles appear fanlike and enter 

 into all three lobes, where they terminate. 



The central part of a muscle band is usually 

 occupied by one or two radial nerves, although 

 muscles witliout a central nerve (figs. 83 and 84) 

 do occur. 



The contraction of the radial muscles pulls the 

 entire mantle inside and throws its surface into 

 ridges. Such a general reaction usually precedes 

 the contraction of the adductor muscle and the 

 closing of the valves. The contraction may 

 occur spontaneously in response to some internal 

 stimulus or it may develop as a result of external 

 irritation produced by chemicals, mechanical and 

 electrical shock, or sudden change in illumination. 

 In response to a weak outside stimulus only a 

 small sector of the mantle contracts, making a 

 slight V-shaped indentation along its periphery. 

 This response may or may not be followed by 

 contraction of the adductor muscle. Strong 

 stimuli, as a rule, result in complete withdrawal 

 of the mantle, contraction of the adductor muscle, 

 and closing of the valves. Besides the large 

 radial bands there are many smaller bundles of 

 transverse fibers (fig. 77, tr.m.) extending diag- 

 onally across the thickness of the mantle, a well- 

 developed system of longitudinal muscles (l.m.), 

 and the oblique muscles (ob.m.) of the tentacles. 



The longitudinal or concentric muscles foUow 

 the general outlines of the edge. They are more 

 abundant at the thickened distal edge of the 

 mantle but do not exhibit the definite pattern 

 of distribution apparent in the radial muscles. 

 The transverse muscle fibers are more numerous 

 in the pallial curtain (fig. 77, tr.m.) than in the 

 other parts of the mantle. They are so arranged 

 that the position of the curtain may be quickly 

 changed in response to external or internal stimuli. 



All the muscle cells are of the smooth, non- 

 striated type with typical elongated nuclei. In 

 some bivalves the nmscle fibers of the mantle 

 appear to slmw a double obli((ue striation; tliis 

 was shown to be an optical effect created by a 

 series of fine fibrillae spiralling around tlie larger 

 fibers (Fol, 1S8S; Marceau, 1904). Muscle fibers 



THE MANTLE 



83 



