NATURAL HISTORY OF THE SHIP-WORMS. 209 
After this shifting, which takes place as the young ship-worm begins to bore, these 
muscles no longer are sharply defined, but form wide bands which run from the 
apophyses to be distributed around the sides of the foot 
In ship-worms the posterior end of the body, which has usually been described as 
the “ muscular collar,’ contains a number of highly specialized muscles, some of which 
are peculiar to the ship-worms. Their general arrangement and their relations to 
the pallets, to the calcareous tube, and to the siphons are shown in figures 35 and 
36. They are divided into two sets, those which manipulate the pallets and those 
which are distributed to the siphons. The first set consists of a pair of protractors 
of the pallets (pp), two pairs of retractors of the pallets (rp), and a single adductor 
of the pallets (ap). On each side the protractor of the pallet is inserted along 
the handle of the pallet, whence it radiates to be attached to the side of the cal- 
careous tube along a broad line, its origin. On each side there are two retractors of 
the pallet. One is inserted on the end of the handle and passes forward to be dis- 
tributed in the mantle along the sides of the body. The other is inserted near the 
outer end of the handle, whence it runs forward to be attached to the wall of the 
calcareous tube along with the siphonal muscles. The adductor of the pallets is a 
stout, cylindrical muscle stretching between the anterior ends of the two pallet 
handles, and lying in the septum which divides the mantle cavity into epi and hypo 
branchial cavities posteriorly. The muscles of the siphons are attached on each 
side to a triangular area of the calcareous tube, slightly anterior and ventral to the 
attachment of the pallet muscles. From this origin the siphonal muscles are 
distributed to the siphons, mostly to the respiratory. 
The action of the muscles of the pallets and siphons is as follows: When the 
ship-worm is undisturbed, the siphons are widely extended into the water and the 
pallets are drawn forward, as represented in figure 35. If the animal is disturbed 
the siphons are retracted with great rapidity by the contraction of their muscles. 
At the same time, by the action of the protractors of the pallets, the pallets are 
pushed forcibly into the end of the tube so as completely to close the latter. The 
outer ends of the paddles are brought together by pushing against the sides of the 
tube. The pallets are dislodged by the more powerful ventral retractors, and 
retraction seems to be completed by the long muscles attached to the ends of the 
handles. At the same time, by the action of the adductor of the pallets, their 
paddles are separated so as to permit the extension of the siphons by an inflow of 
blood. 
From this description it is seen that the end of the tube of Teredo is homologous 
with the pallial sinus of typical lamellibranchs. The same siphonal muscles are 
present as in other forms, but the muscles of the pallets are peculiar to the 
ship-worms. 
RESPIRATORY SYSTEM. 
\ 
The gills of Teredo are perhaps more highly specialized than those of any other 
type of lamellibranch, for they possess a membranous, nonperforate portion which 
reminds one of the gill structure in the Septibranchia, and they are otherwise 
sharply marked off from those of forms most nearly related to the ship-worms and 
of other lamellibranchs. 
