Meaning and Origin of Foot:
Phylum Mollusca is characterised by the pronounced development of musculature known as the foot. It is the locomotory organ in Molluscs. This organ is quite uncommon and strange to others. It is regarded as the remnant of the ‘dermo-muscular tube’ of the ancestral form whose ventral side became greatly developed as an adaptation for creeping movement and the dorsal portion became degenerated.
Origin:
In Mollusca, the foot originates at first as the ventral or ventro-lateral elevation of the ectodermal cells behind the mantle emerging in Veliger and some other larval forms, later the mesodermal cells incorporate to give it a definite shape.
Innervation:
The foot and its associated structures are innervated by the pedal ganglia and pedal nerve cord.
Depending on different modes of locomotion and living in varying environment, the foot in Molluscs varies greatly in shape and form (Fig. 16.63). Variation of foot is primarily due to various physiological activities like creeping or crawling, burrowing, leaping, looping, swimming, reproduction, etc. Besides these, in parasitic and sedentary forms, the modification of foot occurs in the form of sucker, byssus apparatus, etc.
Structure of Foot:
In most typical gastropods, particularly in prosobranchs the foot is usually differentiated into:
(1) A small anterior propodium,
(2) A large middle mesopodium and
(3) A posterior metapodium (e.g., Natica, Fig. 16.65D).
(A) Primitive foot:
The primitive and simplest form of foot in Mollusca is considered to be a broad ventral flat sole having the above mentioned three regions (e.g., Polyplacophora, Gastropoda, Bivalvia [Protobranchia]). Beside these they adapt certain structures as parapodia and epipodia.
Parapodia:
Lobe-like lateral extentions given out from below upwards from the edge of the ventral sole (e.g., Aplysia), and act as fins.
Epipodia:
Projecting paired ridges or folds developing from the sides or base of the foot along its entire length. These may be beset with papillae or tentacles (e.g., Fisurella).
These parts are variously modified in different species of gastropods.
Modification of Foot:
Foot-as the Creeping or Crawling Organ:
Class Aplacophora:
True molluscan foot is absent in Aplacophorans but some structure may be regarded to be the starting point. The ventral foot in Chaetoderma, Limifossor (Subclass Chaetodermomorpha) is absent.
In Neomenia (Subclass Neomeniomorpha), a mid-ventral groove from mouth to anus with non-muscular ciliated ridge is believed to be homologous with the foot of other molluscs and serves as locomotory organ (Fig. 16.65U). The foot helps to glide or to creep over the substratum with a mucous trail.
Class Polyplacophora:
In Polyplacophora, the foot of Chiton is broad, muscular and flattened that extends the entire ventral surface of the body (Fig. 16.65A). In Chitonellus and Crypsoplax the foot is narrow. In Ischnochiton the anterior portion of the foot is elongated.
The foot helps to creep or glide on the rocky substratum by the waves of muscular activity which is lubricated by mucus glands. It also helps to be attached firmly to the rocks by generating a suction, secreting the mucus along the girdle.
Class Monoplacophora:
In Neopilina and Vema the foot is centrally placed, broad, flattened and almost circular in outline. The foot helps in creeping by muscular movement.
Class Gastropoda:
In most gastropods the foot is an elongated, flat creeping sole that contains numerous mucus-producing gland cells. In the members of the subclass pulmonata the foot is undivided with a very large flat lobe containing a large pedal gland. In these cases the foot is used for creeping on a mucous trail. The terrestrial pulmonates retain the primitive type of foot.
The locomotion of most pulmonates is accomplished by the monotaxic waves, i.e., the amplitude of the wave proceeds across the entire width of the foot. In prosobranchs, Patella has a well-developed ventral foot with a flat creeping sole which is adapted for clinging or moving over the rocks (Fig. 16.65C). The creeping foot may be contractile as in Triton. In some cases foot shows partial regional modification.
In Pirulus only the left part of the foot acts as creeping organ. In Acteon (Fig. 16.65K) and Cypraea foot has a large creeping sole. They move by producing waves of contraction on the foot.
In Bullia (Fig. 16.65R) the foot is peculiar and encircles the whole of the body. In Atlanta the posterior part of the creeping foot is altered into a sucker. In Haliotis (Fig. 16.65B), the epipodium is well- developed with many small tentacles (sensory in function).
The flat sole of Murex (Fig. 16.65S) and the highly glandular foot of Conus (Fig. 16.65G) with a long backwardly bent siphon are efficient creeping organs. In Caecum, the creeping movement is performed only by the action of cilia present in the ventral surface of the foot. The bubble snail, Bulla crawl rapidly on the surface of soft bottom as their foot is widely extended on all sides.
Mechanism of Locomotion:
Typical creeping movement in molluscs, especially in gastropods, is brought about either by muscular activity or by a combination of ciliary and muscular activity. Muscular activity of the foot during creeping movement is effected by a series of wave-like contractions of the longitudinal muscles of foot.
The waves of contraction may be monotaxic, i.e., the wave spreads along the entire width of the foot or ditaxic, i.e., the wave spreads only half of the width of the foot and the animals are progressed by advancing alternately the right and left sides of the foot.
The wave of muscular contraction may be direct, i.e., the wave is moving in the same direction as the movement of the animal or may be retrograde, i.e., when the pedal wave passes from forwards to backwards.
In most of the molluscs the amplitude of the waves is small but in some gastropods as exemplified by Helminthoglypta dupetithouarsi (Fig. 16.64) the amplitude as well as wavelength are increased during galloping motion.
During galloping motion the anterior portion of foot is elevated and thrust forward (Fig. 16.63). Foot gets the nerve supply from the pedal ganglion.
Bivalvia:
The foot in Nucula and Area are considered as primitive type, which possess a flat, ventral surface of sole on which the animal creeps.
Foot—as the burrowing organ:
In some molluscs foot becomes greatly deviated to act as burrowing organ.
Class Scaphopoda:
In Dentalium the foot is conical, trilobed and protrusible (Fig. 16.65H). In Siphonodentalium, the foot terminates in a retractile disc with papillated margins. The foot of the Scaphopoda is adapted rowing habit in sand and the co foot is buried into sand and lateral the foot, epipodial lobes assist in burrowing.
Class Gastropoda:
In Terebra, the extremity of foot with flow of blood is extended and acts as anchor. In Natica (Fig. 16.65D), Polinices, Sigaretus (Fig. 16.65P), the propodium is demarcated from the rest of the body by deep transverse grooves as a semicircular flap and the metapodium is provided with lateral parapodium.
The animals have adapted for burrowing on soft bottom habitat. The propodium acts like a plough and anchor, and a dorsal flap-like fold of the foot protective shield. In Harpa the propodium is separated by a constriction.
Class Bivalvia:
In Anodonta and Unio, the foot is triangular and plough-share like. The foot can perform the effective burrowing organ in addition to acting as a creeping organ. In Solemya, Yoldia the foot has a flattened sole and two sides of the sole can be folded to form a blade-like edge which can penetrate into the mud or sand and act as soft bottom burrowers.
In Pholas foot assumes a short and blunt form. In Mya (Fig. 16.65I) the foot is feebly developed and used as a weak burrowing organ. Donax has a thin pointed foot. Tagelus, a razor clam possesses an elongated foot which acts as burrower. The foot of Solen and Ensis is large and cylindrical, and can be outstretched into a wide sheet of Muscles as it plunges into the sand.
In most bivalves, the foot is laterally compressed and blade-like, and the anterior part of the foot acts as a burrowing organ in the soft substratum where they live.
Mechanism of burrowing:
Foot movement is effected usually by the muscle actions of pedal retractors and protractors in a combination with blood pressure that acts as a hydrostatic skeleton.
Foot is extended forward into the mud or sand by the extension of the protractor muscles, and the retractor muscles help to draw the animal deeper into the substrate. The retractor muscles contract sequentially, so the shells rock forward and backward and the animal progresses through the sediment.
Foot—as the leaping organ:
This type of modification is very characteristic in certain forms of Molluscs. In Cardium (Fig. 16.65L), the foot is bent upon itself as a leaping organ. In Trigonia it is compressed antero-posteriorly as an elongated keel. In Poromya, the foot is curved with well- formed protractor and retractor muscles. In Mytilus, the cylindrical foot acts as spring- tail. A median triangular outgrowth in Birabia acts as leaping organ.
Foot—as a looping organ:
In Pedipus, the propodium of the foot is sharply marked off from the rest by a groove and helps in looping movement.
Foot in sessile forms:
A number of sessile bivalves such as sea mussels (e.g., Mytilus, Perna, Modiolus) or oysters (e.g., Ostrea, Crassostrea) are attached to the hard substrates (e.g., rock, corals, shells, wood, sea walls, jetties and pilings, etc.) either by byssus or by the one valve to the substrates. Byssus is a bunch of fine silky threads by which the bivalves are attached to the objects.
The byssal threads are proteinaceous secretions from byssus gland of the foot. The members of mytilids use the byssus for attachment to other objects. The foot is elongated and not used for the lead of sessile life.
Pen shells (e.g., Pinna, Atrina) are attached to the underlying solid substrate by byssus and partly remain buried in sandy sediment. Oysters are firmly cemented to the rock and other substrates. They lack foot and byssal threads.
Crepidula (class Gastropoda) is a sedentary animal which possesses the reduced foot, and the ventral sole acts as an effective sucker for attachment. Vermicularia (Gastropoda) a worm-shell snail is a sessile gastropoda and its foot is reduced. The shell is attached to solid substrates such as rocks and other shells, or entangled in sponges.
Foot—as the swimming organ:
In many Molluscs the foot becomes variously modified to swim in water. The development of parapodia is observed in many forms. Carinaria (Fig. 16.65N), Atlanta and Pterotrachea swims by a flat ventral fin which bears a small sucker representing the original foot. The parapodia is fan-like in Aplysia (Fig. 16.65J) and used for pelegic existence.
It may have wing-like processes as in Pteropoda (Fig. 16.65Q). In Oxygurus the parapodia are hollow and possess fin-like outgrowth. In Atlanta, the metapodium is produced into laterally flattened swimming lobe. In Clione (Fig. 16.65E) the parapodia are well-developed and are placed on the lateral sides of the anterior end.
The most notable transformation of the foot as swimming organ is observed in cephalopods. The foot is partly modified into muscular arms and tentacles around head for food capturing, adhesion and locomotion, and partly into a ventral siphon for jet propulsion.
The cuttle fish and squids move backwards and not forwards on the basis of jet-propelled mechanism. The siphon (= funnel) squirts water forward, pressure of the action squirts the animal simultaneously backwards.
The siphon (also called funnel or in-fundibulum) is a ventral, muscular, sub-conical tube helps in swimming via jet propulsion by expelling water from the mantle cavity.
The expulsion of water is achieved by muscular contraction of the siphon or funnel. In Nautilus (Subclass Nautiloidea) the funnel consists of two separate lateral muscular lobes that fold together to form a tube-like structure which serves for jet propulsion. The funnel of Nautilus is not a complete tube.
In subclass Coleoidea (e.g., cuttle fish, squids) the siphon is a complete one and is made up of two muscular lobes which are completely fused. In both groups the funnel is used for swimming.
Arms and tentacles:
Surrounding the mouth there are a number of short, muscular arms and retractile tentacles which are the derivative of molluscan foot. The number of the oral arms varies in different groups. In the orders sepioidea (e.g., cuttle fish, sepiolids, spirulids) and teuthoidea (true squids) there are 8 arms and 2 tentacles. In Vampyromorpha and Octopoda there are 8 oral appendages only but no tentacles.
When the arms are long and retractile, they are called tentacles and when the arms are short, heavy and non-retractile, they are called arms. The tentacles of cuttle fishes and squids are used for prey capture and the arms are also used for grasping the prey and also help in locomotion via jet propulsion.
In subclass Nautiloidea (e.g., Nautilus) the circumoral appendages are from 63 (males) to 94 (females). The oral appendages are arranged in three separate groups known as ocular, digital and buccal.
The ocular tentacles occur as pair in each side of the head. They are chemosensory in function. The digital and buccal tentacles are arranged as two more or less complete rings surrounding the mouth. The arms do not bear suckers.
In subclass Coleoidea, the head bears 8 non-retractile, sucker bearing arms, and many in addition bear two longer retractile tentacles between the 3rd and 4th pair of arms, e.g., all cephalopods except Nautilus. The tentacular arms (e.g., orders Sepioidea and Teuthoidea) are provided with suckers only at their tips which are club-shaped or spatulate used for clinging to the objects.
In order Octopoda (e.g., Octopuses) and Amphitretus (Fig. 16.65T) have a web developed between the arms, and use this umbrella-like skirt for swimming.
Thus a membrane unites-4 arms (e.g., Tremoctopus), 6 arms (e.g., Histioteuthis) and all arms in Amphitretus, to form a umbrella-shaped structure, helps in swimming. The basal portion of some or all the eight arms are united by a membrane, helps in swimming.
Boring molluscs:
Bivalves that bore into mud (Ark family), wood (Barnea, Bankia, Martensia, Teredo), limestone (Lithophaga, Saxicola), sand stone (Pholas), rock (Petricola), and also coral and coralline rock, possess elongated and cylindrical shells. The foot of the boring species is not well developed.
Mechanism of boring:
The boring is done by the anterior end of the bivalve shells. The anterior end of the valves are usually serrated and have abrading surfaces. The drilling is a mechanical process and is done by the anterior ends of the valves which are rotated back and forth by which the species can bore into the hard substrates. The boring species can attach to the substrate with the help of foot which has developed a sucker-like ventral surface.
Foot—as an organ helping reproduction:
One or more arms in some male cephalopods during breeding season are modified for the transference of spermatophores to the female, called hectocotylus. In Sepia the fourth left arm of the male becomes hectocotylus in which some basal rows of suckers are suppressed. In Loligo and Octopus the third right arm is modified to form hectocotylus. The modifications may involve suckers, calimus and ligula, etc.
Accessory glands associated with the foot:
The foot in Molluscs is also a highly glandular structure. Some glands become intimately associated with the foot to help in locomotion. The secretion of the glands lubricate the passage during movement. In Gastropods the pedal glands and the unpaired sole gland are the typical instances.
In some Bivalves, in adult or in larval stages the byssus apparatus helps in the process of adhesion. The Organ of Valenciennes in some Cephalopod females, Van der Hoeven in Nautilus males are the other notable accessory organs associated with the foot.
Parasitic forms:
The foot of the parasitic forms of molluscs like Entoconcha, Entocolax, Enteroxemes, Thyone (live within sea cucumbers echinoderms), Eulima, Stylifer (endoparasites of echinoderms), is either reduced to small appendages or completely absent.