400 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



all sides, the wliole 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 Yoldia majhe divided into two chief groups : («) trans- 

 verse and (6) 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 w^hole siphon wall outside 

 and inside is covered by a jelly-like tissue, containing nuclei, and at places showing 

 elongated cell- walls, c. 



THE DIGESTIVE TRACT. 



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 {Nucula and Yoldia). In other cases they may be prolonged as 

 narrow bands (iHyt^?ws) or they may be short and thick (Os^rea). The first case is 

 shown in Fig. 52, PI. lxxxvii, at j>, where the palp extends fii^om the mouth back to 

 the gills {(i) 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 may be 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 i^osterior 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 iDosition before the 

 mouth. A dorso-veutral 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 fartliest away fi'om their 

 attached basal portion (Fig. 97, PI. xciv, ip). The folds at this ft-ee 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 occur the 

 secondary folds. The bases of the folds rest upon a more or less complete connective 



