THE PROTRACTOR PEDIS MUSCLE. II 



The idea that the small muscle passing inwards from the " spot " — which in position 

 is a good deal anterior to the functional adductor — might be the reduced remains of the 

 larval anterior adductor, is at once negatived by the observation that the fibres do not pass 

 through the body from side to side, as adductor fibres must ; but that we have to do with 

 two distinct muscles, one on each side of the body. This, then, suggests the muscles for 

 moving the " foot," and although there is no foot in the adult oyster, there can be no doubt 

 that Ryder and Jackson were right in referring to this as a " pedal " muscle. 



The muscles which may be present in other Lamellibranchs in relation to the foot 

 are a protractor and an anterior and a posterior retractor on each side. The posterior 

 retractor is out of the question. The protractor and the anterior retractor are usually 

 inserted into the shell a little posterior to the anterior adductor muscle, and the protractor 

 is the more ventral ly placed of the two. The hinge line of the oyster is anterior, and if we 

 may judge from Horst's figure of the larval oyster at the stage when the foot, as a ciliated 

 ventral projection between the mouth and the anus, is in its most fully developed state, a 

 protractor muscle would run posteriorly from in front of the mouth or oesophagus ; while a 

 retractor would, from what we know of it in other molluscs, probably be placed more 

 dorsally, so as to cross the stomach. Our muscle runs antero-posteriorly, and is towards 

 the ventral part of the visceral mass, close to the insertion of the labial palps. Such 

 indications as we have, then, point to the " spot " being the place of insertion of the modified 

 remains of the protractor pedis muscle. 



The foot is however lost in the oyster at a very early period — while the larva is still 

 free-swimming ; and if the protractor muscle is present in the adult in this relatively large 

 condition, it must be because it has become adapted to a new purpose. We have traced 

 the muscle fibres in the American oyster by means of serial sections from the insertion into 

 the shell (PI. II., Figs. 9 and 10) at the " spot," posteriorly along the top or attached edge of 

 the outer labial palp, to the anterior end of the branchial lamellae. The fibres first run 

 inwards, and then obliquely backwards (Fig. 11), and come into close relation with a large 

 blood sinus (see PI. II., Figs, n, 12, 13, lac), which lies in the mantle just over the attachment 

 of the outer labial palp. The muscle then works its way round the sinus, so as to appear 

 upon its ventral, and finally its inner face (Fig. 12). It thus comes to lie internal to some of 

 the renal tubules, and close to the outermost caeca of the liver. Our muscle now crosses at 

 right angles a bundle of muscle fibres running from the mantle downwards ventrally into 

 the palp, and containing a large nerve, as is usual with the muscle bundles of the 

 mantle. Finally, having reached the level of the posterior end of the palp, our muscle 

 turns ventrally, and begins to spread out (Fig. 13) in the sub-epithelial tract of mesodermal 

 tissue which lies between the top of the mantle lobe and the attachment of the gill lamellae, 

 and which is morphologically a part of the stem of the ctenidium. Most of the fibres end 

 in the connective tissue, immediately under the epidermis, between the descending (internal) 

 and the ascending (external) lamellae of the outer gill. The whole course of the muscle is 

 about 6 mm. long in a moderately large oyster ; it is about l'5 mm. in diameter 

 where inserted into the shell, and less than half that size when it reaches the branchiae. 



