356 ROBERT TRACY JACKSON ON THE 



is on the dorsal border of the adductor, as in Mya and Modiola, PI. xxv, figs. 9-10. The 

 fibres of the posterior retractor pass forward to their insertion in the foot and mci-ge 

 with the byssal muscle as distinctly seen from the left side. An anterior retractor of 

 the foot, «?■, figs. 1-3, is inserted in the left valve near the umbonal region. Whereas in 

 the j^oung the adductor, ])osterior retractor of the foot and byssal muscles are distinct 

 and separate, in the adult they are rendered ambiguous by being in close contact with 

 one another, although making three separate scars in the left valve of the shell. The 

 true nature of the muscles of the adult, as described by Lacaze-Duthiers, is therefore 

 seen to be clearly apparent in the young. 



In young Anoviia attached to glass slides whei-e the action of parts may be observed 

 through the very translucent valves, it is noticeable what an active part the byssal and 

 retractor muscles of the foot take iu pulHng down or closing the upper, left valve, on ac- 

 count of being attached distally by means of a calcareous union to the object of support. 

 This is rendered clear by com2:»aring PI. xxix, figs. 1-2 with the diagram fig. 3. By the 

 action of these muscles the upper left valve is brought in close contact with the object of 

 support, so that it conforms to the siirface as does the shell of a Crepidula, or limpet, under 

 somcAvhat comparable conditions of fixation. In correlation with this changed function 

 of the retractor muscles of the foot, and byssal muscle \\\ Anomia glabra, it is important 

 to notice that the adductor muscle is very small and the lower right valve extremely thin, 

 a mere veil of tissue in the young, so that it would be almost fuuctionless for protection, 

 if the young could be exposed to attack from this side, which however is not the case. 

 It is thus evident that the byssal muscle and the anterior and posterior retractors of the 

 foot in the young are together pseudo-functionally an adductor in their action and as a 

 correlation a reduction of the actual adductor and lower valve has taken place. This con- 

 dition of afiairs is evidently brought about by the position of the parts relatively to the 

 surroundings and is ascribed to them. Lacaze-Duthiers ascribes the absence of retrac- 

 tor muscles of the byssus and foot on the right side to atrophy and loss from disuse. 

 The great size and amalgamation of the byssal muscle and posterior retractor he as- 

 cribes to the changed directions of the fibres and the functional activity in the changed 

 position, perpendicular to the object of leverage. He may have intended the compari- 

 son of the muscles to an adductoi- which I make, but it is not so understood from his 

 text. 



The young Anomia opens its shell by the lifting of the upper valve alone and it has 

 considerable forward and backward as well as lateral motion due to its peculiar method 

 of attachment. "When quietly feeding, the anterior retractor of the foot, ar, PI. xxix, 

 figs. 1 and 3, may shorten and this muscle being inserted at the byssal region of calca- 

 reous fixation the animal responds by moving backwards ; conversely, if the posterior re- 

 tractor, pr, figs. 1 and 3, shortens, the shell is moved forwards, as will be understood 

 by considering the diagram, fig. 3. In the adult there is little or none of this forward 

 and backward motion, as the byssal plug is closely suri-ounded by shell growth and also 

 the position of the posterior retractor is changed so that it could hardly bring about 

 such a movement. 



The foot in adult Anomia glahra is a small reduced organ; but in the young it is very 

 large and active, PI. xxix, figs. 1, 2 and 5. The base of the foot is marked by a deep 



