Biology Reference
In-Depth Information
2.8.1.4 Alimentary canal
The larval oesophagus leads from the posterior end of the pharynx and
terminates near the caudal tip of the liver as it unites with the anterior
intestine (Fig. 11); the latter eventually extends into a posterior intestine
and a hindgut that empties into a cloaca. A new oesophagus develops
during metamorphosis from the epithelium covering the dorsal ridge of the
pharynx, but this oesophagus is totally independent of the pharynx (Fig. 11).
This arrangement is necessitated by the tidal ventilation (pumping in and
out of the branchiopores) that is required for the suctorial feeding habit. The
features of development of this new oesophagus have received considerable
attention in early literature (for review see, Youson, 1980, 1981a). Detailed
light and electron microscopic studies have taken place in
P. marinus
during
the seven stages of metamorphosis of the developing adult oesophagus and
the changing larval oesophagus (Elliott, 1989). Evidence is provided that
the most anterior region of the larval oesophagus plays some role in the
formation of the adult oesophagus (Fig. 11). A summary and description of
the formation of the new oesophagus and the alteration of the junction of
the oesophagus and anterior intestine is provided in Fig. 12. The remainder
of the larval alimentary canal undergoes a complete transformation that
increases the surface area for absorption by the development of longitudinal
Figure 11.
Diagrammatic representation showing the forward movement of the oesophagus-
intestinal junction and the development of the adult oesophagus (AO) from a dorsal cord
(DC) of tissue during metamorphosis in
P. marinus
. a. larva; b. metamorphosing stage 3;
c. metamorphosing stage 4; d. juvenile. Whereas the larval oesophagus (O) leads from the
caudal end of the pharynx (Ph), the adult oesophagus is independent of the pharynx. A and B
denotes regions of the larval oesophagus and how it contributes to development of the adult
oesophagus and intestine (I) and a small diverticulum (D). (From Elliott, 1989).
