HORNELL ANATOMY OF PLACUNA 63 



larger surrounding the former except on the posterior aspect. The posterior section 

 may be said to be sunk in the anterior one (fig. 9). 



The posterior component in most preserved specimens retains a darker colour than 

 the other and larger region ; it extends forwards from the posterior margin of the mass 

 for about three-fourths of the diameter of the entire adductor, and so is covered 

 dorsally and ventrally by arm-like prolongations of the anterior region. Little 

 histologieal difference can be noticed between the elements of the two sections except 

 that the fibres of the anterior appear rather finer, and are aggregated into larger and 

 more densely compacted bundles than is the case with those of the posterior. The 

 muscle bundles round the periphery are large and wedge-shaped in section ; those of 

 the centre small and rounded. Both the component masses are freely permeated with 

 blood spaces more numerous, however, in the hinder section of the muscle. In 

 Pecten the fibres of the anterior component show an appearance of striation ; here 

 no indications of this could be perceived in sections the fibres of both regions 

 appear quite smooth. 



The blood supply is derived from the posterior limb of the aorta through one 

 artery entering the adductor on its upper surface and by three others entering between 

 the halves of the parieto-splanchnic ganglion (fig. 15). Innervation is directly from the 

 ganglion just named, whence two trunks arising side by side from the two halves of the 

 mass pass immediately into the substance of the muscle, dividing as they go into small 

 twigs (fig. 10, N.add. and fig. 29, Par.sp.g.). 



The LEVATOR MUSCLE of the foot is weakly developed in Placuna ; it consists of a 

 narrow band of muscle fibres inserted obliquely in the left valve immediately posterior 

 to the ventral extremity of the anterior hinge tooth (fig. 19, Lev'.). Thence the fibres 

 pass vertically downwards, spreading laterally as they approach the base of the foot, 

 where they blend with the intrinsic pedal muscle fibres. A similar reduction of the 

 pedal levators to an unpaired weak bundle inserted in the dorsal region of the left valve 

 is met with in Anomia. 



The PEDAL RETRACTORS are similarly reduced in Placuna to a single fairly stout 

 bundle, which arises from the ventral part of the muscular base of the foot and passes 

 downwards to the left side of the dorsal surface of the adductor, where it finds insertion 

 upon the left valve (fig. 31). In Placuna this muscle has little importance owing to 

 the absence of a byssus ; large development of the pedal retractors appears to be 

 correlated with special development of a byssal organ. As examples we have two 

 powerful retractors present in Margaritifura mdgaris, where a many stranded byssus 

 exists, while in Anomia, where the byssus undergoes modification into a great calcified 

 organ, these muscles are still larger and more important, with the unique peculiarity of 

 being both inserted in the left valve in like position as the single one in Placuna. In 

 view of this special function these muscles might with greater propriety be termed 

 byssal, and not pedal retractors. 



