376
N. BALAKRISHNAN NAIR AND M. SARASWATHY
Teredo and Nausitora) there is a conspicuous bulbular portion containing within it a highly coiled typhlosole (Fig. 17). This is absent in
Teredora princesae. According to Purchon (1960) it probably serves as
a valve preventing the passage of undigested particles of wood from
the appendix into the mid-gut, adding that the tension that would
develop on the stomach wall when the stomach and appendix are filled
would force some of the stomach contents into the mid-gut. I n such
instances this typhlosole probably acts like a stopper preventing the
entry of stomach contents into the narrow part of the mid-gut.
The anal canal is closed in Bactronophorus, Neoteredo, Dicyathifer,
and Teredothyra and is open in all other genera of timber boring
Teredinidae (see Turner, 1966).
8. The ctenidia
The ctenidia of shipworms are considerably reduced (Fig. 18).
According to Turner (1966) they are represented by the inner demibranch only but a vestige of the outer demibranch is found in some
species. Ridewood (1903), Atkins (1937) and Morton (1958) also consider this single demibranch as the inner one though Purchon (1939)
thinks it to be the outer demibranch in both Xylophaga and Teredo.
Each ctenidium consists of homorhabdic branchial filaments inserted
uniformly on a vascular axis of connective tissue traversed by muscle
fibres. The interfilamentar junctions are organic indicating a synaptorhabdic condition and the intrafilamentar junctions are vascular
representing a typical eulamellibranch condition. The nature and
extent of the ctenidia vary considerably in different genera. For
example in Teredora and Uperotus the ribbon-like ctenidia extend from
the base of the siphons to the labial palps with a well-developed
marginal groove. I n others the ctenidia are much smaller, consisting of
an anterior section of a few ascending filaments (five in Teredo navalis,
seven in Teredo megotara, eight in Nausitora hedleyi, nine in Bankia
indica and ten in Banlcia minima and in Teredo norvegica) separated
from the major part of the ctenidium situated behind the visceral mass.
The two sections are connected by a branchial groove passing along the
dorso-lateral side of the caecum. In Banlcia gouldi Sigerfoos (1908)
considers this groove as formed by the great broadening of the 10th or
11th filament during post-larval development. When compared to the
ctenidia of other shipworms, those of the adult Nausitora hedleyi are
comparatively short and the posterior section lies far back in the mantle
cavity away from the caecum in full grown specimens. The progressive
displacement of the ctenidia in Nausitora hedleyi during the growth in
length of the body is shown in Fig. 19. In transverse sections through
N. BALAKRISHNAN NAIR AND M. SARASWATHY
Teredo and Nausitora) there is a conspicuous bulbular portion containing within it a highly coiled typhlosole (Fig. 17). This is absent in
Teredora princesae. According to Purchon (1960) it probably serves as
a valve preventing the passage of undigested particles of wood from
the appendix into the mid-gut, adding that the tension that would
develop on the stomach wall when the stomach and appendix are filled
would force some of the stomach contents into the mid-gut. I n such
instances this typhlosole probably acts like a stopper preventing the
entry of stomach contents into the narrow part of the mid-gut.
The anal canal is closed in Bactronophorus, Neoteredo, Dicyathifer,
and Teredothyra and is open in all other genera of timber boring
Teredinidae (see Turner, 1966).
8. The ctenidia
The ctenidia of shipworms are considerably reduced (Fig. 18).
According to Turner (1966) they are represented by the inner demibranch only but a vestige of the outer demibranch is found in some
species. Ridewood (1903), Atkins (1937) and Morton (1958) also consider this single demibranch as the inner one though Purchon (1939)
thinks it to be the outer demibranch in both Xylophaga and Teredo.
Each ctenidium consists of homorhabdic branchial filaments inserted
uniformly on a vascular axis of connective tissue traversed by muscle
fibres. The interfilamentar junctions are organic indicating a synaptorhabdic condition and the intrafilamentar junctions are vascular
representing a typical eulamellibranch condition. The nature and
extent of the ctenidia vary considerably in different genera. For
example in Teredora and Uperotus the ribbon-like ctenidia extend from
the base of the siphons to the labial palps with a well-developed
marginal groove. I n others the ctenidia are much smaller, consisting of
an anterior section of a few ascending filaments (five in Teredo navalis,
seven in Teredo megotara, eight in Nausitora hedleyi, nine in Bankia
indica and ten in Banlcia minima and in Teredo norvegica) separated
from the major part of the ctenidium situated behind the visceral mass.
The two sections are connected by a branchial groove passing along the
dorso-lateral side of the caecum. In Banlcia gouldi Sigerfoos (1908)
considers this groove as formed by the great broadening of the 10th or
11th filament during post-larval development. When compared to the
ctenidia of other shipworms, those of the adult Nausitora hedleyi are
comparatively short and the posterior section lies far back in the mantle
cavity away from the caecum in full grown specimens. The progressive
displacement of the ctenidia in Nausitora hedleyi during the growth in
length of the body is shown in Fig. 19. In transverse sections through