TEREDO. 217 



play loose in the anterior part of the tube, they are kept in 

 proper position by the powerful operation and suction of the 

 foot, and 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 branchiae, which are 4^ 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 throiigh the long canal ; these pallets act as a sort of 

 force-pump, and operate thus : When the branchiae 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 communica- 

 tion between the branchial and anal tubes, it follows that the 

 effete 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 com- 

 munication 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 

 compartment of the mantle ; it must either be poured forth 

 by the anterior gape or branchial siphon--! believe from 

 both indiscriminately. 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 ; the 

 branchial orifice is usually white, sometimes pale red with 

 eight or ten short terminal cirrhi, which are furnished with 

 minute retractors springing 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 



