412 



HISTOLOGY 



sec 



la,. 



structure, it should be examined in a form in which the organ is not so 

 highly developed, and where the cellular relations are more easily made 

 out. 



The byssus gland of the mollusk, ^Enigma <znigmatica, as described 

 by G. C. Bourne, is a very favorable object and Figure 374 shows part 



of a section through the byssus 

 gland that shows the most impor- 

 tant features. This structure is a 

 deeply invaginated gland, extend- 

 ing in from the surface through 

 a duct, which widens inside into 

 a chamber of some size. This 

 chamber is partly divided by a 

 median septum into two lateral 

 halves, and from the upper walls 

 a series of thin lamella hang 

 down, forming a laminated gland. 

 The secretion is produced by at 

 least two sets of cells. The first 

 part appears as a product of cells 

 situated in the deep upper parts 

 of the lamellar acini. This mate- 

 rial is moved, as a thin sheet or 

 lamina of the peculiar hyaline bys- 

 sus substance, down towards the 

 edges of the lamella or folds of tis- 

 sue bearing the cells that produce 

 it (Fig. 374, secx.). 



The middle zone of these la- 

 mellae bears flat cells whose sur- 

 face is beset with short, stiff cilia. 

 Some writers have denied that 

 these cilia moved, and others have 

 even disclaimed the idea that 

 they were cilia. The writers be- 

 lieve them to be short, strong cilia which move the lamina of material 

 downward. These cells secrete no material (Fig. 374, cilx.}. 



When the laminae reach the distal edges of the lamellae they are added 

 to by the epithelial cells of this edge which are enlarged and crowded 

 full of " bissageneous " granules. Figure 374 shows this condition well. 

 The various laminae now fuse, hi some cases remaining single, and form 

 the well-known threads which attach the mussel to the rocks. 



In other mollusks, as Anomia, the byssus substance is formed by 



FlG. 374. Vertical section through the edge 

 and upper part of one of the lamellae of the 

 byssus gland of ^Enigma, la., lamina of se- 

 cretion at sec.; ctt.c., ciliated cells lining the 

 middle region of the sides of the lamellae; 

 sec.c., upper or distal secreting cells. (After 

 G. C. BOURNE.) 



