THE BIOLOGY OF WOOD-BORING TEREDINID MOLLUSCS
371
caecum of the Adesmacea, Purchon (1955) concludes that these are
homologous and provide reliable evidence of relationship.
The bent caecum of Teredora (Fig. 15) and Uperotw is probably
associated with the need for the storage of wood fragments since in
these genera the elaborate typhlosole is not present. A rudiment of the
typhlosole is evident in the caecum of Teredora princesae. In Nototeredo norvagica a typhlosole is apparently absent. In Neoteredo the
caecum is of moderate size. Species having large caeca occur in the
highly specialized genus Nausitora where this organ is noteworthy not
only for its size but also for its contained greatly coiled typhlosole.
Similarly in both the advanced genera Bankia and Teredo the caecum
is large with a doubly coiled typhlosole. Thus this organ seems to be
an index of specialization. The larger and more specialized the appendix, the higher its place in the evolutionary scale. Nevertheless, the
reasons for the development of an elaborate typhlosole which is obviously a device for increasing the area of absorption within the caecum
is difficult to understand in the context of the present concept of the
function of the caecum as an organ merely for the temporary storage of
wood fragments. However, in the light of the recent emphasis on the
role of bacteria and fungi in the digestive process of wood-boring bivalves it is possible that‘the digestion of at least a part of the wood
occurs within the caecum.
It is not clear how the wood stored in the caecum finds its way into
the stomach and then into the tubules of the digestive diverticula. The
wall of the caecum does not appear sufEciently muscular t o force the
contents out and it is difficult to understand the method by which the
stored fragments emerge. Purchon suggests (quoted by Reid 1966) that
“ the stored particles may be flushed out by the closure of the midgut
entrance and the contraction of the stomach.” According to Reid
(1965), by their contraction the adductor muscles expel the contents of
the caecum as a mucus bound mass. It is possible that the movements
of the animal within the burrow also play an effective role in this
process.
There is an apparent relationship between the elaboration of the
cwcum and reduction of the labial palp. For example : (1) the simple
caecum without the coiled typhlosole of Nototeredo norvagica and its
large palps capable of selective sorting of food materials ; (2) the large
caeca of Nausitora, Teredo and Banlcia and their inconspicuous palps
incapable of quantitative selection.
Since the terms “ liver ” or “ hepatopancreas ” imply an assumption regarding their homology and since they are digestive in function,
the mass of tubules round the stomach are known as digestive
A.M.B.-9
13
371
caecum of the Adesmacea, Purchon (1955) concludes that these are
homologous and provide reliable evidence of relationship.
The bent caecum of Teredora (Fig. 15) and Uperotw is probably
associated with the need for the storage of wood fragments since in
these genera the elaborate typhlosole is not present. A rudiment of the
typhlosole is evident in the caecum of Teredora princesae. In Nototeredo norvagica a typhlosole is apparently absent. In Neoteredo the
caecum is of moderate size. Species having large caeca occur in the
highly specialized genus Nausitora where this organ is noteworthy not
only for its size but also for its contained greatly coiled typhlosole.
Similarly in both the advanced genera Bankia and Teredo the caecum
is large with a doubly coiled typhlosole. Thus this organ seems to be
an index of specialization. The larger and more specialized the appendix, the higher its place in the evolutionary scale. Nevertheless, the
reasons for the development of an elaborate typhlosole which is obviously a device for increasing the area of absorption within the caecum
is difficult to understand in the context of the present concept of the
function of the caecum as an organ merely for the temporary storage of
wood fragments. However, in the light of the recent emphasis on the
role of bacteria and fungi in the digestive process of wood-boring bivalves it is possible that‘the digestion of at least a part of the wood
occurs within the caecum.
It is not clear how the wood stored in the caecum finds its way into
the stomach and then into the tubules of the digestive diverticula. The
wall of the caecum does not appear sufEciently muscular t o force the
contents out and it is difficult to understand the method by which the
stored fragments emerge. Purchon suggests (quoted by Reid 1966) that
“ the stored particles may be flushed out by the closure of the midgut
entrance and the contraction of the stomach.” According to Reid
(1965), by their contraction the adductor muscles expel the contents of
the caecum as a mucus bound mass. It is possible that the movements
of the animal within the burrow also play an effective role in this
process.
There is an apparent relationship between the elaboration of the
cwcum and reduction of the labial palp. For example : (1) the simple
caecum without the coiled typhlosole of Nototeredo norvagica and its
large palps capable of selective sorting of food materials ; (2) the large
caeca of Nausitora, Teredo and Banlcia and their inconspicuous palps
incapable of quantitative selection.
Since the terms “ liver ” or “ hepatopancreas ” imply an assumption regarding their homology and since they are digestive in function,
the mass of tubules round the stomach are known as digestive
A.M.B.-9
13