Describe the respiratory system of Pila.
Ans. Pila exhibits double mode of respiration i.e. it can absorb oxygen
dissolved in water by ctenidium and can also utilise atmospheric oxygen by
the pulmonary sac.
The mantle cavity is incompletely divided into right chamber (branchial)
and left chamber (pulmonary) by the presence of epitacnis. Aquatic respiration
is performed by the single ctenidium situated on the dorso-lateral wall of the
right portion of the mantle cavity. The ctenidium is made up of numerous
triangular leaflets or lamellae. These lamellae are arranged in single row running
parallel to one another along the ctenidial axis of the gill. This type of the
ctenidium is known as the monopectinate type. The basal end of each lamella
is attached to the pallial epithelium and the other end hangs freely branchial.
The lamellae are not of same size. The lamellacau large in the middle of the
ctenidium, while the lamellae decrease in size towards the two ends. Each
branchial lamellae is composed of two layers of epithelia supported by muscle
fibres and connective tissue. Two epithelial layers enclose a narrow space,
Each epithelial layer consists of three of cells : (i) ciliated columnar cells,
(ii) non-ciliated columnar cells and (iii) few glandular cells. The detailed
structures of a gill lamella is shown in Fig. The ctenidium is supplied with
blood vessels. In aquatic respiration, a current of water containing oxygen is
drawn in by the left siphon into the mantle cavity which ultimately flows into
the right side by the help of epitaenia to bathe the ctenidium. The nuchal lobes
help in the process. After the exchange of gases the water is expelled from the
mantle cavity through the right siphon.
Fig. Respiratory organs in Pila.
A. The mantle is partially displaced to show the position of ctenidium
B. Transverse section of a branchial lamella.
C. Diagramatic sectional view of the outer wall of the pulmonary sac.
The Pulmonary sac is a closed cavity which hangs from the dorsal wall of
the mantle in the pulmonary chamber. The pulmonary sac in Pila is a new
attainment in response to its aerial respiration. The pulmonary sac has one
opening into the pulmonary chamber which is guarded by two valves. The
wall, specially the dorsal wall of the pulmonary sac, is highly vascular and
helps directly in gascous exchange. Pila can also respire through the pulmonary
sac while it remains in water.
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