58 OKHAMANDAL MARINE ZOOLOGY REPORT 



than 1'75 cms. in diameter. What the maximum may be is difficult to say in respect 

 to fully adult animals, but I have note of an instance where on opening a living 

 specimen suddenly, and with a minimum of shock, the foot was seen to be bent 

 downwards, and to lie along the ventral side of the adductor muscle with the sucker 

 approximated to the anus. In this state of extreme extension it was somewhat reduced 

 in diameter ; the length was slightly over six centimetres. 



The substance of the foot is composed of networks of muscle fibres running in 

 various directions, the principal being longitudinal and radial. Others again are dis- 

 posed concentrically, and by these various series the foot can be deflected as required. 



A weak levator muscle (fig. 4, Lev'.), difficult to discern without the aid of serial 

 sections, originates in the upper portion of the pedal base, and assists in retracting the 

 foot upwards, its insertion being in the left valve posterior to the ventral end of the 

 anterior cardinal tooth, while a similarly weak muscle bundle passes from the ventral 

 pedal base to the upper surface of the adductor, and is therefore the homologue of the 

 powerful retractors seen in Margaritifera. In Placuna a single bundle alone is 

 traceable, its insertion being in the left valve at the anterior dorsal corner of the 

 adductor muscle (fig. 31, -Ret.), as is also the case with the two retractors present 

 in Anomia. 



Sections of partially extended feet show the muscular framework to be so 

 extensively penetrated by blood cavities as to be highly cavernous. Injection of these 

 spaces with blood produces extreme inflation or turgescence, which, as seen, enables the 

 organ to attain an elongation at least four times that of the contracted condition, and 

 in conjunction with the complex of muscles composing it and acting upon it, provides 

 for all the varied movements of which the foot is capable. 



The blood supply is derived from a stout artery given off from the anterior region 

 of the hepatic visceral artery. Blood passing from the foot enters the left visceral 

 sinus and passes to the kidneys and thence to the gills. 



The nerve supply to the foot comes from the pedal ganglion, which is situated 

 close to and practically within its base on the dorsal side. A single nerve trunk is given 

 off which ramifies throughout the tissue of the foot in a most complete manner 



Both byssus and byssal gland are absent, neither is there any trace of an otocvst 

 the latter a peculiarity shared with Solenomya and a few other Lamellibranchs. 



The inner surface of the pedal sucker appears to be homologous with the locomotor 

 region of the foot in Margaritifera : whether the byssal gland is also represented by 

 the interior of the sucker cannot be decided until we obtain embryological light upon 

 the subject. 



