Foot and shell of molluscs

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1. FOOT AND SHELL OF MOLLUSCS -BY: KRITHIGA .G.R (20ZBT49 )

2. CONTENT PREFACE FOOT : Includes class Monoplacophora,Amphineura,Gastropoda,Pelecypoda,Scaphopoda,Cep halopoda FUNCTIONS OF FOOT SHELLS AND ITS STRUCTURE,TYPES OF COILING. COMPOSITION OF SHELL,VARIATION IN SHELLS. REDUCTION OF SHELL FUNCTIONS OF SHELL REFRENCES

3. PREFACE: The Mollusca ( Lat.Mollis,soft ) as implied by their scientific name, are mostly Soft-bodied animals. They are also called as Shell fish. FOOT : On ventral side of the body, there is a flat, elongated, muscular surface which is termed as foot of the Mollusks. It aids in locomotion & adhesion. The flat surface has production of mucous which helps in movement of animal . SHELL: In all Mollusks, which posses a shell, this is secreted by mantle which is composed of CaCO3.It is somewhat smaller in male than in female. While some of the common slugs lacks shells, so they are termed as naked .

4. CLASS: MONOPLACOPHORA MONO-ONE/SINGLE ,PLAK=A PLATE They are also termed as deep sea limpets or Trybilidia GENERAL CHARACTERS: 1.Length=3mm to 35 mm, resembles like elf-hats. 2. Bilateral symmetrical, considered as long extinct fossil lineage until 1952,found in Costa Rica (deep/shallow waters) FOOT : The foot of Neopilina / Vema is centrally placed ,broad and round in outline .The flat sole which helps the animals to creep.

5. CLASS:AMPHINEURA SUB-CLASS: APALCOPHORA Foot is reduced or absent except in order Neomeniophora In the order Neomeniophora ,there is a presence of vestigial foot. It is a small longitudinal ciliated ridge . SUB –CLASS:POLYPLACOPHORA Foot is broad and greatly expanded ,forming a large flattened sole used for locomotion and for firm contact with root surface. Foot is narrow in case of Chitononellus . There is a secretion of mucous for proplusion .

6. CLASS-GASTROPODA DIVISION OF THE FOOT : The foot has been variously modified. There are 3 divisions of the foot. * PROPODIUM : A small anterior portion * MESOPODIUM : A large middle portion *METAPODIUM : present posteriorly The anterior margin of foot forms small tactile papillae (trochus) or protrude small fleshy process called mentum( turbonilla ).

7. APPENDAGES OF FOOT : The Gastropoda has many appendages of which epipodia and parapodia are most important. EPIPODIA: prominent folds arising from the sides of the foot along its entire length. PARAPODIA: It refers to the lateral fleshy protrusions .

8. GLANDS ASSOCIATED WITH FOOT The foot contains scattered unicellular as well as aggregated multicellulae mucus-secreting pedal glands. This includes 1 ) LABIAL GLANDS- in groove at the anterior end. 2) SUPRA-PEDAL GLAND : between head and anterior end of foot 3) SOLE GLAND : opening mid-ventrally in the middle of foot . 4 ) POSTERIOR GLAND- at the hind end of foot .

9. MECHANISM OF MOVEMENTS LEAPING: Some gastropods living on the ground not always move in a crawl. Conches also use their sabre-shaped operculum. Their operculum dug into the ground, they catapult themselves forward, so they jump over the ocean floor. When they are threatened ,they can also use their operculum to deal painful blows. SWIMMING: In swimming and sessile forms, however the foot is greatly reduced or greatly modified. They move by the beating action of cilia of the foot on the mucous sheet secreted by the anterior part of foot. OPERCULUM: When the animal is completely withdrawn into the shell ,its aperture is closed effectively by a flat ,oblong plate called operculum. CREEPING: Creeping movement in gastropods is brought about either by muscular activity or by combination of ciliary and muscular activity. Muscular activity of the foot during creeping movement is effected by series of wave like contractions of the longitudinal muscles of foot. BURROWING: The mechanism used in burrowing by gastropods also exhibits convergence with burrowing Bivalvia. Burrowing by gastropods is essentially a continuation of surface locomotion obliquely into soft substrata ,as occurs in Naticidae,Olividae .

10. SESSILE GASTROPODS AND PARASITIC GASTROPODS: SESSILE GASTROPODS : In sedentary and clinging forms like Patella ,the foot is reduced and the ventral sole acts as sucker. In worm shells like Vermicularia ,shells are immobile,either they are attached to other shells or entangles with sponges. PARASITIC GASTROPODS : Parasitic gastropods ,like Stylifer , Thyca and Entoconcha , endoparasitic in echinoderms,show reduction or loss not only of foot but of their other body organs

11. CLASS-PELECYPODA The animals moves forward by the contraction of longitudinal muscles. There also termed as Hatched foot. TRIGONIA&CARDIUM : Foot is large, narrow and it is bent. When straightened functions as creeping organ. MYA : Foot is poorly developed TELLINA : This bivalve has a large muscular foot for rapid burrowing as the tide goes out. It can change the shape of the foot with blood pressure to pierce the sand . ANIMALS WHICH ARE SEDENTARY: 1. PECTEN,ANOMIA,CHAMA: -Foot is rudimentary. 2. OSTREA: -Foot is absent 3. MYTILUS :Presence of byssus gland ,helps to secreate bundles of filaments that function to attach the mollusc to solid surface .

12. CLASS-SCAPHOPODA The class Scaphopoda has a body plan that is bilaterally symmetrical and the foot is modified for digging and burrowing. There also called as Shovel foot. STRUCTURE OF FOOT : It is large, pointed spade like structure that arises from the ventral end of the shell. EXCEPTIONS: 1IN .DENTALIUM : The foot is conical ,trilobed structure and bears ridge/pleat on both sides 2.IN SIPHONODENTALIUM : Foot ends in a retractile disc with papillate margins 3.IN PULSELLUM : It has a filiform tentacle in the middle of the disc. RIDGE : A linear elevation.

13. CLASS-CEPHALOPODA They are named so because their head is situated directly on top of their foot.The foot is remarkably modified into 8-10 circum-oral arms . These arms help to capture the prey and Ventral funnel which helps in respiration. SUB-CLASS : IN DECAPODA(SEPIA,LOLIGA ): There are 5 pairs of arms which bears stalked suckers and 4 th pair of arm is called as TENTACTLE IN OCTOPODA (OCTOPUS ): There are 4 pairs of arms bearing sessile suckers CIRCUM -ORAL ARMS: Located around the mouth and holds stinging cells which are used to inject prey with venom.

14. FUNCTIONS &MOVEMENT OF FOOT FUNCTIONS OF FOOT: 1 . CRAWLING: Foot helps in crawling and creeping movement. 2.REPRODUCTIVE: In cephalopods, the arms become hectocotylised in males ,which acts as intromittent organ. 3 LEAPING : * In cardium , foot contacts & extends and acts like spring. *In birabia , there is a triangular out-growth which act as leeping organ. 4. LOOPING: Often the propodium of the foot is sharply marked off from the rest of the body by a groove. Eg : Pedipus They also helps in Swimming and Burrowing . MOVEMENT OF FOOT : The foot has become adapted for locomotion. The movement of the foot is brought by the combination of blood pressure and muscular action. During movement ,the foot swells due to rush of blood and action of protractor muscles produces extension. The contraction of retractor muscles and the fibres of foot causes with drawl of foot.

15. SHELLS-INTRODUCTION PREFACE: The soft body of the snail is protected by a thick globose, lemon yellow ,brown or even –blackish shell. It is somewhat smaller in males than females. A single revolution of the shell round the axis is called as WHORL . Totally there are 61 whorls ,which increases in size from the summit ( APEX ) to base( BODY ). The last or lowermost whorl encloses greater part of the body. SUTURE acts as common link between two whorls. The surface of the shell is marked by numerous lines of growth or fine ridges called VARICES.

16. STRUCTURE OF SHELL: PERIOSTRACUM: It is the thin outermost layer ,composed of uncalcified ,homogenous horny organic material called CONCHIOLIN . It is lemon-yellow to greenish brown in color which is easily rubbed off. OSTRACUM : It is thicker and has calcareous plates which runs at right angles to the margin of the apex. Each plate is surrounded by protein matrix.It is also known as Prismatic. NACRE : The calcareous plates are arranged parallel to the margin of the aperture ,each deposited over a thin organic matrix, forming thin lustrous sheets or lamella. It is also called as mother-of-pearl/ hypostracum . The microscopic structure of snail ,reveals it has three layers.

17. TYPES OF COILING : COILING: In snails, the shells are spirally coiled .Inside the snail ,heart and shell are aligned to one-side of the body. There are two types of coiling: 1. DEXTRAL : This means that the body, including shell is spiralled towards right or in clockwise direction as viewed from apex of the snail. 2. SINISTRAL : This means that the body ,including shell is spiralled towards left or in anticlockwise direction as viewed from the apex . THE COILING PHENOTYPE that is seen in the offspring is controlled by the genotype of the MOTHER .

18. COMPOSITION OF SHELL

19. VARIATION IN SHELL IN DIFFERENT CLASSES SHELL IN NAUTILOIDEA : The nautiloides appeared first in Cambrian period. The shell is straight or coiled with sutures SHELL IN CEPHALOPODA : During the course of evolution ,they developed from the nearly straight nautiloid and their shell become completely enclosed. SHELL IN SEPIA : It has only the septa but the shell and thickened parts of the wall have disappeared. SHELL IN OCTOPUS: The shells have been completely disappeared during the course of evolution/

20. REDUCTION OF SHELL: The reduction of shells can be seen in gastropods . It also overgrows and completely encloses it or even completely disappear as seen in nudibranchs. The many possible benefits of shell loss include weight reduction , coping with mineral deficiency , streamlining , an ability to crawl into tight spaces , a more distensible body , and the exposure of the skin to the outside world . 1)Great tendency of shell reduction is observed in Opisthobranchia . 2)In many cases ,the lateral folds of the mantle are reflected over the shell and come closer to enclose it completely. 3)In slug, shell is much reduced 4)In some cases shell is reduced ,becomes thin and horny and partly concealed as in case of Dolabella .

21. FUNCTIONS OF SHELLS : Shell function as protection for the snail from predators It prevents from mechanical damage. Also protects from dehydration It also serve as muscle attachment and calcium storage. The snails breathe through breathing hole which is under the shell.

22. REFRENCES https://www.sciencedirect.com/science/article/ https://www.ck12.org/book/ck-12-biology-advanced-concepts/section/15.19/ Shell loss in mollusc evolution (andrewgray.com) Foot & shell of Mollusca by Kotpal Foot & shell in Mollusca by Verma Textbook of Zoology by Nicholoson