SEA MUSSEL MYTILUS EDULIS. 1 43 



there is a thick layer of gland cells whose contents stain deeply with Delafield's hsema- 

 toxylin. A second type of gland cell lies peripherally to these just below the epithe- 

 lium or, in some cases, in the epithelium itself, as single cells filled with round granules 

 which stain deeply with Bordeaux red or eosin. 



The epithelium of the oesophagus is a continuation of that of the lips, but in the 

 transition the surface changes from a smooth to a convoluted condition. The epithelium 

 is of the columnar ciliated type similar to that on the lips except that in the basal region 

 the gland cells increase greatly in number as the stomach is approached (fig. 125, MG, 

 opp. p. 143). They are the type of gland cell which stains with hematoxylin. There 

 are also small cells of irregular form containing yellow granules which lie scattered 

 throughout the basal region of the epithelium in varying numbers. List (1902) thinks 

 they are probably amoebocytes which were caught while wandering through the epithelium 

 of the oesophagus where they were loading themselves with food material to be carried 

 to assimilation organs in the manner suggested by the researches of Carazzi (1893) 

 on the oyster. 



The epithelium rests on a distinct basal membrane, below which there is a thin 

 layer of circular and longitudinal muscle fibers. According to Sabatier (1877) the 

 circular fibers form an inner layer in relation to the longitudinal muscles, but in the 

 author's preparations the two sets of fibers seem to be intermingled. 



The stomach has its inner surface thrown into numerous folds and prominences of 

 various sorts in different individuals. Sometimes it gives rise to one or more pockets 

 from the dorsal, lateral, or ventral walls. 



The epithelium of the stomach is similar to that of the oesophagus except that 

 it is higher and almost without any mucous gland cells (fig. 123, opp. p. 142). The 

 cells are all ciliated, but the cilia are proportionately shorter. In many places, chiefly 

 on the dorsal wall, they may be entirely covered with a homogeneous substance very 

 similar to the crystalline style (fig. 124, CS, opp. p. 142) . The basal membrane on which 

 the cells rest varies in thickness in different places, being more strongly developed 

 where the greatest folds appear and showing scattered nuclei lying within its substance. 



Circular muscles lie immediately below the basal membrane, and, according to 

 Sabatier (1877), external to these there is a layer of longitudinal fibers. This layer, 

 however, the author could not demonstrate to his own satisfaction. 



The direct intestine or tubular stomach, in the terminology of Sabatier (1877), 

 arises from the posterior end of the stomach as a canal which at first is round in cross 

 section, becoming oval in the middle region, the large end usually placed ventrally 

 (fig. 128, opp. p. 143). Sometimes it is found in a dorsal position. Often the lateral 

 walls of the canal project inward so as to divide the direct intestine into a dorsal and 

 a ventral canal. In each case the larger lumen contains the crystalline style, the smaller 

 one performing the usual functions of an intestine. The epithelium of the two regions 

 is very different, that which lines the walls of the lumen occupied by the crystalline 

 style being composed of low columnar cells covered with relatively heavy cilia, while 

 that of the intestinal portion is lined with a columnar ciliated epithelium whose cells 

 are much higher. The lateral walls form thick folds of very high cells which also 

 carry a heavy coat of cilia. The wall lying opposite to the crystalline style cavity is 

 lined with a relatively low columnar ciliated epithelium between the cells of which 

 lie numerous gland cells of the beaker type. 



