MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 395 



more scattering- foot muscles. This very generally occurs in forms with this kind of 

 foot. In Toldia and Solenoniya,iiliio,t]ie sexual gland occupies a considerable j)ortion 

 of the upper part of the foot. The walls of the foot are made of a more dense layer 

 of muscle fibers (Fig. 13). 



In such forms as Mytilus, where the foot is degenerated and is of little or no use to 

 the adult animal as a burrowing organ, it is not necessary that it should be expanded, 

 and it has almost entirely lost its blood spaces, those only remaining which may con- 

 tain blood for the nourishment of the tissues of the foot. The fibers are closely packed 

 together, making the foot very dense and tough. 



THE BTSSUS. 



This organ is generally considered to be a gland of the foot. The byssus itself is 

 made of a number of horny secreted fibers which attach by their outer ends to foreign 

 objects. The part winch does the secreting, the byssus organ, occupies various posi- 

 tions in different lamellibrauchs. In Rucula there is a small blind sac near the poste- 

 rior edge of the disc of the foot, and Pelseneer has described above this several gland- 

 like cells which he thinks represent the byssus organ. In Venericardia borealis a 

 well-developed byssus is present in a slight groove near the middle of the ventral 

 surface of the foot. In MytiluH the great byssus has no connection with the foot in 

 the adult, but is situated behind it. 



The byssus organ of Yenerieardia is one of the best for examination, as it is not 

 greatly complicated. Fig. 73, PI. lxxxix, represents a horizontal section through it. 

 The secreting surface is deeply folded {fd)^ and in these folds the secretion is seen in 

 long sheets (bs). Surrounding the folds is a mass of the secretion made of concentric 

 layers, which have been added to its inner border. At the inner ends of the folds are 

 many vertical muscles (bm) which are strongly inserted and serve by their attach- 

 ment to the valves of the shell above as a very powerful support. Among these 

 muscles are many large, almost clear cells (c). 



Fig. 74 shows the epithelial surface of one of these folds. The greater num- 

 ber of the lining cells approach to a columnar form {cc), and appear to be ciliated. 

 At the deej)er part of the fold the lining cells suddenly become very large, indistinct 

 {7c), and almost entirely unstained. I could make out no nuclei in them. The cells 

 which seem from the appearance of the section to do the secreting, are the eiiithelial 

 cells {cc). The byssus secretion (bs) I never found extending down over the large 

 clear cells, but it more often adhered to the other cells as shown in the figure, and 

 was much thicker at the outer than at the inner end. In some sections the dense 

 apparent ciliation of these cells suggested that possibly there was really a striated 

 secretion instead of a ciliation. 



If individuals of Mytilus, which have been torn from their attachment, are i)ut 

 in a dish of sea water, they soon become again attached.* A very fine, transparent 

 thread appears, and where it strikes a solid body, its end spreads out in a number of 

 root-like i)rocesses which form a firm attachment. Soon another similar thread appears 

 and attaches itself, generally at some distance from the first. Though this may take 

 place in a minute or two after the individual is placed in the dish, I was not able to 

 see the exact manner in which the thread was protruded. I imagine, however, that 



* Since the above was written, I have carefully observed the method of attachment in ilytilus, 

 which is accomplished by the foot, as referred to by Prof. Verrill. An account of it will be included 

 in a later paper. 



