MORPHOLOGY OF LAMELLIBRANCHIA.TE MOLLUSKS. 393 



The muscle system ofMytilus eduUs is worthy of remark. Fig. 42, PI. lxxxv, repre- 

 sents the imiscnlature dissected out entire. From the base of the byssus at its ante- 

 rior portion, two cylindrical muscles, called the anterior foot-retractors, run forward, 

 passing on either side of the mouth (Fig-. 33, rtr),and are attached to the valves of the 

 shell. They are, in the adult, entirely free from any muscles connected with the foot 

 and all their fibers are inserted in the base of the byssus. The tongue-like foot, with 

 its concave anterior surface (Fig. 42,/), is continued directly upward to the shell, as 

 two cylindrical muscle bundles (Fig. 42, fm), which pass exteriorly to the two anterior 

 nmscles just mentioned (Fig. 36, fm and ar). The foot with these muscles may be 

 removed without disturbing any of the other muscles. These are the posterior foot- 

 retractors. The byssus organ is morphologically a part of the foot, and these muscles 

 des(;ribed are probably the anterior and posterior foot-retractors, respectively. But 

 the byssus organ has lost all connection with the musculature of the foot, as have the 

 anterior adductors also. 



From the base of the byssus, just behind the insertion of the two anterior muscles, 

 extends a great mass of muscle bundles which, attaching themselves to the shell above 

 and posteriorly, serve as its main support. These byssus muscles are arranged in two 

 groujjs. They are close to one another at the byssus and diverge laterally above to 

 become attached to the valve of either side. (Shown in section in Fig. 37 at h)i.) The 

 mass extends obliquely backward and is divided with much regularity into a number 

 of large bundles (Fig. 42 bm). They are shown in the vertical sections at hm in Figs. 

 38 to 40. 



A dorsal muscle, well shown in JSFucula and Solenomya^ occupies a position nearly 

 parallel to the anterior foot-retractors (Pelseneer, No. 17, Figs. 7 and 15), and its attach- 

 ment to the shell is slightly posterior to the latter. In Pecten the foot retractors, and 

 in A )i ami a the byssus muscles, are attached only to the left valve. 



Other large muscles are developed in the mantle of many lamellibranchs, in the 

 region of the siphon, which will be spoken of in that connection. 



THE FOOT. 



This characteristic organ, appearing generally, though sometimes greatly modi- 

 fied, throughout the MoUusca, is in lamellibranchs a muscular projection from the 

 ventral surface of the main body of the animal. It extends more or less anteriorly. 

 In the x)rimitive forms Yoldia and iSolenomya, the lower part of the foot is tui-ned nearly 

 forward, and where the organ has degenerated greatly, as in Pecten irradians, it arises 

 from the extreme anterior end of the visceral mass. In 3fya arenaria it is relativelj^ 

 small, though functional in locomotion and situated far forward. In Venus mercenaria 

 and many others, it occupies the whole ventral surface of the visceral mass, extending 

 slightly backward as well as forward, and in a few forms there is a greatly develojied 

 heel-like projection posteriorly. 



Gertain simple and probably primitive lamellibranchs jiossess a foot which has a 

 more or less circular ventral disc {Pectnnculus, N'uculidcv, Solenomyida?). Around the 

 margin of this are a number of thick, short papillae or flutings. Fig. 52, PI. lxxxvii, 

 representing Yoldia limatula with the right valve and mantle fold removed, will show 

 the general relations of the foot (/). It is seen to have a volume greater than all the 

 rest of the animal, the mantle being very thin. The ventral disc (d) is not expanded 



