Mr. W. Clark on the Plioladidae. 331 



do not require permanent fixation like the smaller pallet end of 

 the animal. 



I will briefly explain the operation of the posterior spatulate 

 appendages in compressing and relaxing the siphons. The very 

 great length of the branchise, which are 4i inches long, out of a 

 total of 8 or 9, together with the extent of the tubular cylindrical 

 mantle, requires an aid to facilitate the flow of water through the 

 long canal ; these pallets act as a sort of force-pump, and operate 

 thus: — When the branchise require water their siphon is filled, 

 and its inflation acted on by the spatulate valves being brought 

 together ; the sphincter is simultaneously relaxed, and the water 

 forced into the branchial cavity, after which it is again closed by 

 the separation of the pallets, and as I have ascertained that there 

 is no communication between the branchial and anal tubes, it 

 follows that the eff"ete water is expelled by the same canal as it 

 entered, by the action of the pallets on the sphincter. This is 

 precisely the operation of the sphincter in Dentalium ; and in case 

 a communication did exist between the two siphons, the anal, from 

 its very inferior calibre, could not discharge the mass of water 

 received by the branchial siphon into the tubular branchial com- 

 partment of the mantle ; it must either be poured forth by the 

 anterior gape or branchial siphon — I believe indiscriminately from 

 both. I think the anal canal is strictly applicable to discharge 

 the water taken in by its own siphon, for rejectamental uses, and 

 in many cases for the emission of ova. 



The siphons, though short, form a muscular texture of strong 

 elastic threads, crossed by others at right angles j the branchial 

 orifice is usually white, sometimes pale red with eight or ten short 

 terminal cirrhi, which are furnished with minute retractors spring- 

 ing obliquely near their extremities from the muscular tissue ; 

 the anal orifice is plain. The foot is a strong muscular mass ; it 

 sends forth two very strong fillets that are fixed to the spatulate 

 ends of the internal crotchets, which support the body, in lieu 

 of the ordinary adductor muscle of bivalves, which is here, as in 

 Pholas, wanting, to the remarks on which we particularly refer. 



I close these observations by stating, that the masses of the 

 foot, medial adductor, and posterior sphincter supply their re- 

 spective parts of the body with the minor muscular threads. 



It is time to inquire about the nervous agencies that stimulate 

 the action of these powerful muscular organs, and we find their vo- 

 lume, as in Pholas, not to be apparently in accordance with them. 

 There are just above the mouth two minute ganglia so nearly con- 

 fluent, that they may be considered as one ; from them, two very 

 slender threads descend to the roof of the anal aperture, distri- 

 buting numerous ramifications to the proper stomach and foot, 

 whilst the main cords pass into the base of that portion of the 



22* 



