THE COMMON MUSSELS. 7 



M. latus the impression on the upper side about the middle of the 

 shell is that of the middle retractor, while the term posterior retractor 

 impression remains with that part of the retractor impression that is 

 continuous with the posterior adductor impression. 



In M . edulis or M. magellanicus the single impression, called that 

 of the posterior retractor, corresponds to the two impressions of the 

 middle and posterior retractors in M. latus. 



General Description of the Organs of M. latus. (Fig. 1.) 

 On opening the valves of the shell and viewing the animal from 

 the ventral side, the body is seen to consist, speaking roughly, of two 

 lateral flaps and a median portion (Fig. 35). The two lateral wings 

 line the shell throughout, and are the mantle-lobes, being spoken of 

 as right or left according to the valve they occupy. The median 

 portion of the body in nearly the whole of the hinder half is a keel- 

 shaped ridge — the mesosonia (mes, Figs. 1, 2, and 43). Going for- 

 ward from the mesosoma, there is first an organ consisting of a 

 central stalk with numerous bristle-like branches, by which the shell 

 is moored to external objects {bys, Figs. 1 and 9 : in these figures part 

 of the byssus is cut off). This is the byssus. In front of it, in the 

 median line, is a muscular organ, the foot, whose shape varies much 

 according to the extent of its protrusion or retraction [ft, Figs. 1 and 

 9) . At the anterior end of the animal inside the mantle there is, on 

 each side, a pair of depending lobes, the labial palps (i Ip and o Ip, 

 Figs. 1, 2, and 28), that nearest the median line on each side being 

 called the inner labial palp, the other the outer labial palp. Between 

 the anterior ends of the labial palps is the mouth (Fig. 1) . The 

 mantle-lobes are connected at their posterior end by a nearly hori- 

 zontal band below the level of the posterior adductor (j m, Figs. 1 

 and 2). This hinder junction is much more extended in M. edulis 

 (j m, Fig. 3), as it reaches down the greater part of the posterior end 

 of the lobes, starting from above the posterior adductor. The junction 

 of the mantle-lobes in front in M. latus extends round the ventral side 

 for some distance. There is a considerable cavity under the mantle 

 at the anterior end, extending in front of the mouth and above the 

 eesophagus between the anterior retractors (s c, Fig. 1). This chamber 

 I have called the supra-sesophageal cavity, and mussels kept out of 

 water are often found to have the foot curved round in front of the 

 mouth and reaching into this cavity, which often retains water. Its 

 walls are lined with a rich plexus of blood-vessels. 



Extending from between the inner and outer labial palps to the 

 posterior junction of the mantle there are on each side of the foot 



