Phylum Mollusca

8/14/2019 Phylum Mollusca

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Phylum Mollusca By:

Keano Lev Eslabon

Aishwarya Isabel Dela Pena

Marie Athena Ybanez


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Definition

The word mollusc is derived from

the French mollusque, which originated from

the Latin molluscus, from mollis, soft.

Molluscus was itself an adaptation

of Aristotles , "the soft things.

The scientific study of molluscs is known as

Malacology.


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Anatomy

The generalized mollusc has a single,

Iimpet-like" shell on top. The shell is

secreted by a mantle that covers the upper

surface. The underside consists of a single

muscular "foot".


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Mantle and

mantle cavity The mantle cavity is a fold in the mantle that

encloses a significant amount of space. Thecavity was probably at the rear in the earliest

molluscs but its position now varies fromgroup to group. The anus, a pairofosphradia (chemical sensors) in theincoming "lane", the hindmost pair of gills and

the exit openings of the nephridia ("kidneys")and gonads (reproductive organs) are in themantle cavity.


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Shell

The mantle secretes a shell that is

mainly Chitin and conchiolin (a protein)

hardened with calcium carbonate, but not

phosphate with the questionable exception

ofCobcrephora except that the outermost

layer is all conchiolin.


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Foot

The underside consists of a muscular foot, which has

adapted to different purposes in different classes. The foot carries a pair of statocysts which act as balance

sensors. In gastropods, it secretes mucus as a lubricant to aid

movement. In forms that have only a top shell, suchas limpets, the foot acts a sucker attaching to the animalto a hard surface, and the vertical muscles clamp theshell down over it; in other molluscs, the vertical musclespull the foot and other exposed soft parts into the shell.

In bivalves, the foot is adapted for burrowing into thesediment

in cephalopods it is used for jet propulsion, and thetentacles and arms are derived from the foot.


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Physiology

A striking feature of molluscs is the use of

organs for multiple functions. For example:

the heart and nephridia ("kidneys") are

important parts of the reproductive system as

well as the circulatory and excretory systems;

in bivalves, the gills both "breathe" and

produce a water current in the mantle cavitywhich serves both excretion and

reproduction.


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Reproduction

The simplest molluscan reproductive system relies

on external fertilization, but there are more complexvariations.

All produce eggs, from which mayemerge trochophore larvae, more complex veliger larvae,or miniature adults.

Two gonads sit next to the coelom that surrounds the heartand shed ova or sperm into the coleom, from which thenephridia extract them and emit them into the mantle cavity.

Molluscs that use such a system remain of one sex all theirlives and rely on external fertilization.

Some molluscs use internal fertilization and/or arehermaphrodites, functioning as both sexes; both of thesemethods require more complex reproductive systems


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Classes of Molluscs

bivalves

gastropods

cephalopods


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va ves(Bivalvia)

30,000 species

including scallops, clams, oysters and mussels.

have a shell consisting of two rounded plates

called valves joined at one edge by a flexible ligament calledthe hinge.

typically bilaterally symmetrical, with the hinge lying in

the sagittal plane.

unique among the molluscs, having losttheir odontophore and radula in their transition to filter feeding.


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va ves(Bivalvia)

Some bivalves are epifaunal; they attach tosurfaces. Others are infaunal; they bury themselvesin sediment. These forms typically have a strongdigging foot. Some bivalves such as scallopscan swim.

The sexes are usually separate, butsome hermaphroditism is known. Bivalvespractice external fertilization. The gonads are

located close to the intestines, and either open intothe nephridia, or through a separate pore into themantle cavity


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Gastropods

(Gastropoda)

more commonly known as snails and slugs,and include those that live in the sea, infreshwater and on land

most highly diversified class inthe phylum Mollusca, with 60,000 to 80,000living snail and slug species.

snails are distinguished by an anatomical

process known as torsion, where the visceralmass of the animal rotates 180 to one sideduring development, such that the anus issituated more or less above the head


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Gastropods(Gastropoda)

Torsion is present in all gastropods, but theopisthobranch gastropods are secondarily de-tortedTorsion occurs in two mechanistic stages.

Courtship is a part of mating behavior in somegastropods including some of the Helicidae. Again,in some land snails, an unusual feature ofthereproductive system of gastropods is thepresence and utilization of love darts.

In many marine gastropods other thanthe opisthobranchs, there are separate sexes; mostland gastropods however are hermaphrodites.


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Cephalopods

(Cephalopoda)

characterized by bilateral body symmetry, a

prominent head, and a modification of the

mollusk foot, a muscular hydrostat, into the

form of arms or tentacles Teuthology, a branch of malacology, is the

study of cephalopods

The class contains two extant subclasses in the Coleoidea, the mollusk shell has been

internalized or is absent; this subclass includes

the octopuses, squid, and cuttlefish


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Cephalopods

(Cephalopoda)

the penis in most male Coleoidea is a long

and muscular end of the gonoduct used to

transfer spermatophores to a modified arm

called a hectocotylus. That in turn is used to

transfer the spermatophores to the female.

in species where the hectocotylus is missing,

the penis is long and able to extend beyondthe mantle cavity and transfers the

spermatophores directly to the female

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Phylum Mollusca