APPLE SNAIL -PILA GLOBOSA SHELL STRUCTURE-PALLIAL COMPLEX-INTERNAL SOFT PARTS

Pila lives in fresh water ponds, rice fields etc. It is an amphibious form. It belongs to

Phylum : Mollusca

Class : Gastropoda

Sub-class : Prosobranchiata

Order : Mesogastropoda.

Shell:

• Shellof Pila is a single piece and hence it is called univalvia.
• It protects the body of the animal.
• The shell shows an axis called columella.
• It is hollow Around this columella the shell is coiled.
• The shell shows 61/2 coils. The coils are separated by suture lines.
• The apex of the shell is first formed part. The last whorl is the biggest.
• It is called body whorl in which the body of the animal lies.
• This body whorl will open out through the mouth. It shows Hps.
• Near this mouth the columella opens out through umbilicus.
• Hence this shell is called umbiiicated shell or perforate shell.
• On this body whorl vertical lines are present .
• They are growth lines. They are called varices.
• If the mouth of the shell is on the right side of the observer when the apex of the shell is upwards, it is called dextral shell.
• If the mouth is on the left side it is called sinistral shell.

Operculum :

• The mouth of the shell is covered by operculum.
• This is a plate like structure secreted by foot glands.
• On the inner side of this operculum a bulge is present.
• It is called Boss.Foot muscles are attached to it.

Microscopic structure of the shell :

In the section of the shell 3 layers can be seen.

1) Periostracum : It is brown in colour. It is made by conchiolin. It is produced by the glands present on the margin of the mantle.

2) Ostracum : It is the middle layer. It is made by alternate layers of CaCO3,and conchiolin. It is also called prismatic layer.

3) Mother of pearl layer (or) Nacreous layer :

It is made by CaCo3. It is white in colour. It is shining layer. The shell is secreted by the Mantle layer.

Inner soft parts:

When shell is removed the inner soft parts can be seen clearly. They are head, foot and visceral mass. They are covered by protected by Mantle

1. Head: It is present on the dorsal side of the foot. It shows the following parts

a) Two pairs of Tentacles : The first pair of tentacles are present on the mouth. They are called cephalic tentacles. The second pair of tentacles shows contraction and expansion The second pair tentacles are considered as true tentacles.

 

b) Eye Stalks : Behind the second pair of tentacles eye stalks are present They bear eyes. These are called ommatophores.

c) Mouth : At the apex of the head mouth opening is present.

d) Pseudepipodia or Nuchal lobes On either side of the head two
pseudepipodia are present. They are present on the left and right sides of the
head. They are helpful to draw in water into the mantle cavity and to send that
water outside.

2. Foot: On the ventral side of the body foot is present. It is broad and sole like. It has longitudinal and transverse muscles. Their contractions will bring locomo­tion. The foot shows mucus glands. They secrete mucus which will help the animal to creep on the land without friction.

 

Mantle Cavity - Pallial complex :

In between the mantle and body a cavity is present called mantle cavity. It has the following structures.

Epitaneal ridge : Starting near the right pseudepipodium a thick ridge will travel upto the posterior end of the mantle cavity. It is chitinous ridge. It will divide the mantle cavity into right branchial chamber and left pulmonary chamber.

Branchial Chamber : It contains the following structures.

1. Ctenidium : It is a monopectinate gill. It is like a comb. All the lamellae are present on one side of the axis. It helps in branchial respiration.

2. Rectum : It is present on the left side of the gill. It opens out through anus on anal papilla.

3. Genital duct: On the left side of the rectum genital duct is present. (In male vas deferens in female oviduct will be present). This duct will open out through genital pore. In the male pila near the male genital pore penis is present.

4. 0sphradium: Near the left pseudepipodium, attached to the pulmonary chamber wall a leaf like osphradium is present. It will test the purity of water.