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 branchize, which are 43 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 branchize require 
water their siphon is filled, and its inflation acted on by the 
spatulate valves beig 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—TI 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 plan. 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 heu of the ordinary 
