134 BULLETIN OF THE BUREAU OF FISHERIES. 



HISTOLOGY. 



If the shell of the sea mussel is broken or cut in cross section, three distinct layers, 

 sharply defined from each other, are visible to the naked eye: An outer, thin, cuticular 

 layer, the periostracum ; a middle violet-colored portion, the prismatic shell; and an 

 inner glossy white or pink substance, which often reflects iridescent colors, the mother- 

 of-pearl, or nacreous, layer. Under high magnification each of these parts shows a 

 characteristic structure. 



The periostracum generally covers the whole outer surface of the shell and extends 

 over the free edge for a short distance over the inner surface where it terminates in a 

 fold of the mantle border. It is a smooth, glossy cuticula, thinnest in the region of 

 the umbo, where it is often completely worn off as a result of exposure at low tide and 

 the action of strong waves and currents. The hinge of the shell is formed from the 

 periostracum which extends over from one valve to the other. In cross section the 

 periostracum presents three layers (fig. in, Per, p. 136). The outer and inner portions 

 consist of a clear, transparent, brownish substance which does not readily stain with 

 any of the ordinary dyes. The middle portion is colorless and has an affinity for plasma 

 stains. In the region of the mantle edge from which it is an outgrowth the layer con- 

 sists of a single layer of cells, but as the outer surface of the shell is approached the 

 cells disappear, leaving a series of cavities to mark the middle zone. The periostracum 

 may, therefore, be divided into three distinct areas: An outer, middle or hollow, and 

 an inner layer. The periostracum is attached to the layer of the shell lying imme- 

 diately below it by means of trabeculae which are embedded in the calcareous substance. 



The blue portion of the shell or middle layer is composed of fine needlelike filaments 

 of calcareous matter closely united into a single structure by an organic matrix of 

 conchiolin (fig. 108, p. 135). When a valve is broken in cross section and examined 

 with a hand lens this layer presents a series of alternate ridges and grooves with glisten- 

 ing surfaces which extend across the shell. With higher magnification it is possible 

 to see that the prisms are long and almost straight and so arranged as to form an angle 

 of about 45 with the outer shell surface (fig. in, PL, p. 136). The pigment, which is 

 more abundant on the peripheral surface of the layer, is deep blue or violet in color and 

 is distributed in the form of parallel bands which run across the prisms at an angle of 

 about 30 (fig. 112, PB, p. 136). Around the ventral and posterior borders of the shell 

 there is no inner nacreous layer present, consequently the prismatic shell lies in direct 

 contact with the outer fold of the mantle edge. 



The nacreous or mother-of-pearl layer covers the inner surface of the shell out to 

 the mantle line as a boundary. It is thickest in the anterior and middle regions and 

 thinnest at the border. This is the only layer which continues to grow in thickness 

 throughout the life of the mollusk. The nacreous and prismatic layers lie in direct 

 contact with each other without any intervening substance to connect them. The 

 structure of the nacre consists of a series of thin lamellae with irregular edges placed one 

 on the other with their surfaces lying horizontal to the surface of the shell. When 

 seen in cross section of the shell under high magnification they appear as fine irregular 

 parallel lines (fig. 1 11, NL, p. 136). If a portion of the nacreous layer is dried for some 

 time or is treated with sodium hydroxide it becomes fragile and has a tendency to break 

 up into flakes whose surfaces mark the line of cleavage between the separate lamellae. 



