400 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
all sides, the whole still within the mantle folds. Fig. 31 represents a section of the 
sipho ns at some distance from the body. 
The muscles of the siphon of Toldia maybe divided into two chief groups : (^trans- 
verse and (b) longitudinal layers. As far as I know, there are no circular fibers. 
Small transverse and longitudinal layers alternate with one another to form the siphon 
walls. Fig. 53, PI. lxxxvii, represents a transverse section of a bit of the siphon walls 
in this form, as being the cavity of the anal and bs the branchial siphon; ss is the sep- 
tum separating them. The transverse layers ( trm ) extend across the walls. Nuclei 
are present most frequently in the central, narrower portion. At the outer edges of 
the layers the fibers separate and spread out to become attached. In this region, 
also, numerous nuclei of the fibers appear. The longitudinal muscles occur princi- 
pally between the transverse layers (Im), but there are also many smaller bundles near 
the point of attachment of the transverse muscles. The whole siphon wall outside 
and inside is covered by a jelly-like tissue, containing nuclei, and at places showing 
elongated cell- walls, c. 
THE DIGESTIVE TKACT. 
The mouth. — The mouth is situated in the median line between the two labial 
palps, and just behind the anterior adductor muscle when it is present. It is not 
sharply marked off from the oesophagus, being a funnel-shaped opening for it. 
The palps. — These are two lips or folds anterior and posterior to the mouth. They 
extend backward on either side toward the anterior ends of the gills. They are often 
large and plate-like ( Wucula and Toldia). In other cases they may be prolonged as 
narrow bands (Mytilus) or they may be short and thick (Ostrea). The first case is 
shown in Fig. 52, PI. lxxxvii, at p , where the palp extends from the mouth back to 
the gills (g) at the posterior end of the visceral mass. Here and in Nucula (Mitsukuri, 
No. 13) the palps possess a long appendage (ap) supposed to aid in the collection of 
food. These maybe protruded to the exterior just below the siphons. 
Fig. 95, PI. xciii, shows the relation of the palps to the mouth, as well as their 
general shape. The figure represents the anterior end of the body of Mytilus, cut in 
a vertical transverse plane just posterior to the mouth. The most posterior, or inner 
palp (ip), extending on either side of the mouth opening, hides the latter, which is 
situated just above mo. The outer palp (op) occupies a similar position before the 
mouth. A dorso-ventral striation is seen to exist over the lower three-fourths of the 
inner surface of the palp (at op), and the surface of the inner palp, opposed to it, is 
similarly thrown into folds or ridges. 
These ridges have very much the same appearance to the unaided eye as the gill 
filaments, and have led many observers to mistake the palps for gills. They are 
simply ciliated ridges occurring on but one side of each palp — that next to the mouth. 
Sections across these folds would differ somewhat in appearance according to the 
regions from which they were taken. Fig. 62, PI. lxxxvii, represents a section across 
the folds of the palp of Ostrea in the ventral region, in this case farthest away from their 
attached basal portion (Fig. 97, PL xciv, ip). The folds at this free edge of the palp 
are generally thicker than at the base, and are thrown into two or three small second- 
ary folds (sf). The ciliated epithelium covering the folds is composed of much elon- 
gated cells, more greatly developed on the side of the fold on which occul the 
secondary folds. The bases of the folds rest upon a more or less complete connective 
