366
N. BALAKRISHNAN NAIR AND M. SARASWATHY
(Purchon, 1941) and Teredinidae which bore into wood. The earlier
theories concerning the method of boring have been summarized by
Miller (1924) whose detailed observations on Teredo established that in
this species the shell is the only tool used in boring, the foot and
cephalic hood serving to hold the animal in position in the burrow while
the shell is moved by the muscles attached to it. Recently Board
(1970) investigated the tunnelling operations of shipworms using the
technique of radiography and made certain interesting observations.
He noted that the boring end of the burrow contains water which the
shipworm removes by forcing it into the grain at the unlined portion of
the wood ahead of it ; and the lubrication of the shell valves is accomplished by small, shrinking movements of the animal within the
burrow. Although many authors have described the boring action,
descriptions vary widely in scope and differ in interpretation of the
events involved. For example most authors agree that during boring
the animal rotates, but Yonge (1963) attributes this rotation to the
attaching foot moving first in one direction, then in the other while
Morton (1964) considers that rotation in all the Adesmacea is caused by
asymmetrical action of the adductors, the shell turning up to 90'
alternately in each direction. It is generally accepted that in all
Adesmacea the boring action is purely mechanical, but the exact mechanisms involved have not been clearly explained. According to Lane
and Tierney (1951) the denticulated shell-valves maintain contact with
the advancing face of the burrow with the aid of turgor developed in
the mantle cavity. This intramantle pressure in Teredo pedicellata is of
the order of 5-17 mm water. In these long, flaccid bivalves it is this
internal mechanism that insures effective apposition of the boring tools
with the blind, boring end of the burrow. The production and maintenance of turgor is the result of the interaction of siphonal musculature, gill cilia and the mantle. Nair and Ansell (1968) have described
the main activities during boring by Zirphaea crispata using techniques
similar to those which have been applied to the study of the burrowing
action and fluid dynamics of bivalves (Hoggarth and Trueman, 1967 ;
Trueman, 1966, 1967 ; Ansell and Trueman, 1967). In the Teredinidae
a new type of movement about a dorso-ventral axis has been perfected
and the shell valves have become greatly specialized as a boring
instrument free from attachment of the siphonal muscles. The adaptational sequence (Fig. 13) involved in this change in function of the
adductor muscles in the Adesmacea has been followed out by Nair and
Ansell (1968). Recent observations of Ansell and Nair (1969) have
shown that wood boring may be achieved by the same movements as
those involved in rock boring, but may also in some species involve
N. BALAKRISHNAN NAIR AND M. SARASWATHY
(Purchon, 1941) and Teredinidae which bore into wood. The earlier
theories concerning the method of boring have been summarized by
Miller (1924) whose detailed observations on Teredo established that in
this species the shell is the only tool used in boring, the foot and
cephalic hood serving to hold the animal in position in the burrow while
the shell is moved by the muscles attached to it. Recently Board
(1970) investigated the tunnelling operations of shipworms using the
technique of radiography and made certain interesting observations.
He noted that the boring end of the burrow contains water which the
shipworm removes by forcing it into the grain at the unlined portion of
the wood ahead of it ; and the lubrication of the shell valves is accomplished by small, shrinking movements of the animal within the
burrow. Although many authors have described the boring action,
descriptions vary widely in scope and differ in interpretation of the
events involved. For example most authors agree that during boring
the animal rotates, but Yonge (1963) attributes this rotation to the
attaching foot moving first in one direction, then in the other while
Morton (1964) considers that rotation in all the Adesmacea is caused by
asymmetrical action of the adductors, the shell turning up to 90'
alternately in each direction. It is generally accepted that in all
Adesmacea the boring action is purely mechanical, but the exact mechanisms involved have not been clearly explained. According to Lane
and Tierney (1951) the denticulated shell-valves maintain contact with
the advancing face of the burrow with the aid of turgor developed in
the mantle cavity. This intramantle pressure in Teredo pedicellata is of
the order of 5-17 mm water. In these long, flaccid bivalves it is this
internal mechanism that insures effective apposition of the boring tools
with the blind, boring end of the burrow. The production and maintenance of turgor is the result of the interaction of siphonal musculature, gill cilia and the mantle. Nair and Ansell (1968) have described
the main activities during boring by Zirphaea crispata using techniques
similar to those which have been applied to the study of the burrowing
action and fluid dynamics of bivalves (Hoggarth and Trueman, 1967 ;
Trueman, 1966, 1967 ; Ansell and Trueman, 1967). In the Teredinidae
a new type of movement about a dorso-ventral axis has been perfected
and the shell valves have become greatly specialized as a boring
instrument free from attachment of the siphonal muscles. The adaptational sequence (Fig. 13) involved in this change in function of the
adductor muscles in the Adesmacea has been followed out by Nair and
Ansell (1968). Recent observations of Ansell and Nair (1969) have
shown that wood boring may be achieved by the same movements as
those involved in rock boring, but may also in some species involve