SEA MUSSEL MYTILUS EDULIS. * • 1 39 



inner layer of ciliated epithelium, and a middle layer of supporting tissue, gland cells, 

 nerves, and blood vessels. In adult animals this condition is profoundly modified by the 

 development of the genital organs. Preliminary to the growth of the reproductive 

 products the middle tissue layer of the mantle increases greatly in bulk, genital canals 

 proliferate out through it up to the border of the pallial muscles, the blood supply is 

 greatly increased, and fatty tissue is deposited. During the change the mantle increases 

 in size from a thin membrane to a thick fleshy organ. The ratio in mantle thickness 

 before and after the production of genital products may be as much as 1 : 100. 



The free edges of the mantle are different in structure from the rest of the organ. 

 They are firmer and tougher and constitute the region of rigid attachment to the shell. 

 Their edges, which are attached to the ventral and posterior borders of the shell, are 

 divided into three parallel folds that run longitudinally. The inner fold (fig. 1 14, IF, 

 p. 136) is much thicker than the others; anteriorly the edge is smooth, but toward the 

 posterior region it becomes thicker and is fringed with tentacular processes of fimbra? 

 (fig. 117, Fm, opp. p. 138). This region of the mantle edge is dark brown in color, due 

 to the numerous pigment granules which fill the outer portion of the ciliated epithelial 

 cells (fig. 165, PgG, p. 178). The interior of the fold is rich in muscle fibers which 

 give to it considerable contractile quality. When the mussel is resting undisturbed 

 in the water with its shell open, the inner fold of the mantle in the posterior region may 

 extend some distance beyond the edge of the shell. A sudden change in light intensity 

 by casting sunlight or a shadow over it or by applying some slight mechanical stimu- 

 lus will cause it to be withdrawn and the shell closed. 



The middle fold is narrower than the outer one but, like it, is richly supplied with 

 muscle fibers (fig. 114, Mus). Its inner surface and free edge are lined with pigmented 

 ciliated epithelium, while the outer surface is composed of fibers and simple epithelial 

 cells which secrete the periostracum (fig. 114, MF). 



The outer fold is the narrowest of the three. The upper part of its inner surface is 

 covered with tall columnar epithelium, while the rest of its surface is bounded with epi- 

 thelium of the low columnar type. The interior of the fold is richly supplied with 

 muscle fibers. Ordinarily this fold is not visible because of the periostracum, which 

 grows out from the outer surface of the middle fold across to the outer surface of the 

 shell, where it is firmly attached (fig. 114, OF). The inner fold is therefore completely 

 shut off from the exterior. 



The space which is inclosed between the mantle lobes constitutes the pallial or 

 mantle cavity. In it lie the foot, byssus, gills, mesosoma, and the visceral mass. 



DIGESTIVE SYSTEM. 



ANATOMY. 



The alimentary tract of Mytilus edulis presents the characteristic specialized type 

 of digestive organs found in the Lamellibranchia, consisting of an anterior mouth, 

 oesophagus, stomach, a long complicated intestine, and a posterior anus together with 

 two pairs of accessory mouth structures, the labial palps, which serve to convey food 

 into the mouth, and a large digestive gland, the liver. 



The mouth is situated between the anterior retractor muscles of the byssus just 

 posterior to the anterior adductor muscle (fig. 118, Mth, opp. p. 138). When seen from 

 in front or from below it appears as a transverse slit with distinct upper and lower lips 



