MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 
395 
more scattering foot muscles. This very generally occurs in forms with this kind of 
foot. In Yoldia and Solenomya, also, the sexual gland occupies a considerable portion 
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 BYSSUS. 
This otfgan 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 which does the secreting, the byssus organ, occupies various posi- 
tions in different lamellibranchs. In JYucula 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 Mytilus the great byssus has no connection with the foot in 
the adult, but is situated behind it. 
The byssus organ of Venericardia 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 deeper part of the fold the lining cells suddenly become very large, indistinct 
(Ic), 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 epithelial 
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 put 
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 processes 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 Mytilus, 
which is accomplished by the foot, as referred to by Prof. Verrill. An account of it will be included 
in a later paper. 
