36 WALTER K. FISHER, 



which becomes very slender before piercing the epithelium. Most of 

 the cells contain a finely granular secretion which tends to harden in 

 the center of the lumen, leaving a clear space next to the walls. 

 Sometimes the cells are quite empty. In the fully grown animal these 

 glands occur at frequent intervals with glandular spaces between them, 

 but in very young individuals the glands are relatively much enlarged 

 so that there is a continuous cordon of them around the mantle. As 

 the animal increases in size, they tend to withdraw from each other 

 and become segregated as definite bundles of cells. It is probable 

 these glands are concerned with the secretion of material of the shell 

 which consists of several layers. Along the ventral side of the mantle, 

 large gland cells occur at frequent intervals (Fig. L GT). 



The muscular system of the mantle consists of a dorsal and a 

 ventral transverse, or edge to edge, layer with dorso-ventral bundles 

 at frequent intervals. There are also many oblique fibres, but not 

 disposed in any definite bundles. The strong dorsal and ventral layers 

 serve to contract the mantle toward the spindle muscle whenever the 

 animal is irritated. 



The epithelium of the dorsal side is much higher than that of 

 the ventral side and is pigmented. On the left side of the animal 

 the outer fourth of mantle has a zone of rather thicker epithelium 

 on the dorsal side. This epithelium is not thrown into lengthwise 

 wrinkles as is the rest, and contains little pigment. It has been 

 called the mantle sense organ by Haller, who found ganglion cells 

 under the epithelium. 



Along the edge of the mantle are many little sense papillae, sunk 

 in shallow pits lined with pigmented epithelium. Fine branches of 

 the mantle nerves terminate in these papillae. 



Muscular System. 



In this connection only the muscles of the buccal mass will be 

 considered. In life they are bright pink in color, and one can see 

 the reddish tinge through the dorsal wall of the head. This color is 

 in sharp contrast to that of the other muscles of the body, namely 

 the foot, spindle muscle, heart, mantle, et cetera, which, as is well 

 known, are whitish. 



On account of the very definite and rather rapid set of movements 

 in connection with the radula, when the creature is feeding, we find 

 developed in the buccal mass clear cut groups of muscles. In the 

 accompanying figures each muscle has been numbered so that it can 



