SEA MUSSEL MYTILUS EDULIS. .137 



with weak nitric or hydrochloric acid. The staining reactions of the decalcified sections 

 of the ligament are characteristic; the middle layer has a tendency to take up the 

 plasma stains, while the inner layer takes up the basic dyes. 



FORMATION. 



The periostracum is an outgrowth from a specialized portion of the mantle edge. 

 If a cross section of the mantle is examined under 1ow t magnification it will be seen that 

 the edge of the mantle is divided into three distinct folds which run parallel with its edge. 

 They may be designated by their position as the outer, middle, and inner folds. From 

 the outer surface of the middle fold the periostracum arises as a thin cuticula which is 

 secreted bv a layer of epithelial cells having characteristic, long, elliptical nuclei and fibrils 

 which lie at an oblique angle with the surface of the periostracum. Numerous muscle 

 fibers from the mantle terminate among these cells (fig. 1 14, Mus,p. 136). The youngest 

 portion of the periostracum, which lies in contact with the secreting cells, is a thin, trans- 

 parent, homogeneous structure, but as it extends out beyond the limits of the mantle 

 edge it grows progressively thicker and becomes differentiated into the three layers which 

 have already been described. The periostracum grows over the edge of the outer fold, 

 beyond which it becomes attached to the outer surface of the prismatic layer of the shell. 



The prismatic or blue layer of the shell is secreted by the low columnar epithelial 

 cells which cover the outer surface of the outer fold of the mantle edge. As fast as the 

 material is built up along the edge of the shell its outer surface comes in contact with the 

 outgrowing periostracum, to which it becomes attached. 



The epithelium of the outer mantle surface is composed of small cubical cells and 

 gland cells which secrete the mother-of-pearl (fig. 1 14, p. 136). The process is continuous, 

 so that as the animal grows older this layer continues to grow thicker, giving the shell 

 the unusually firm and heavy character which is often noted in old mussels. Exposure 

 to the rough action of waves and currents stimulates the cells to more rapid secretion. 



The ligament arises from a layer of tall columnar epithelial cells which lies imme- 

 diately below it. 



ATTACHMENT TO THE BODY. 



The whole outer surface of the fleshy part of the body is more or less intimately 

 connected with the inner surface of the shell. The epithelium of the mantle forms a 

 rather weak attachment, while the muscles adhere most tenaciously at their points of 

 union with the shell. 



The epithelial cells of the outer surface of the mantle lie in direct contact with the 

 inner surface of the shell and are attached to it by the secretion of a soft, gummy sub- 

 stance from which the shell is being formed constantly. The attachment may be likened 

 to that of a label pasted on a bottle. 



In case of the muscle attachments, a very different type of adhesion is found. 

 Here the epithelial cells of a highly specialized nature serve as anchoring organs. They 

 are so intimately attached to the bundles of muscle fibers at their proximal ends that it 

 is difficult to distinguish them from the contractile tissue without applying staining 

 methods. Ordinarily muscle fibers stain more deeply than do the supporting epithelial 

 cells. Distally the epithelial cells are embedded in the surface of the shell, making an 

 attachment so strong that it is impossible to separate the mass from the shell without 

 applying acid to dissolve away the calcareous substance in which they are firmly fixed. 



