Phylum Mollusca is a very diverse 85, species group of mostly marine species. Mollusks have a dramatic variety of form, ranging from large predatory squids and octopus, some of which show a high degree of intelligence, to grazing forms with elaborately sculpted and colored shells.
This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. Figure 2. This chiton from the class Polyplacaphora has the eight-plated shell that is indicative of its class. These animals lack a calcareous shell but possess aragonite spicules on their epidermis. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia excretory organs. The morphology of the shell and the underlying animal can vary from circular to ovate.
A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in Today, scientists have identified nearly two dozen extant species. These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators.
Respiration is facilitated by ctenidia gills that are present ventrally. These animals possess a radula that is modified for scraping. Eyespots are absent in these animals. A single pair of nephridia for excretion is present. Figure 3. These mussels, found in the intertidal zone in Cornwall, England, are bivalves. Members of this class are found in marine as well as freshwater habitats.
The overall morphology is laterally flattened, and the head region is poorly developed. Eyespots and statocysts may be absent in some species.
These animals are suspension feeders—they eat material, such as plankton, that is suspended in the water around them. Due to their diet, this class of mollusks lacks a radula. Respiration is facilitated by a pair of ctenidia, whereas excretion and osmoregulation are brought about by a pair of nephridia. Bivalves often possess a large mantle cavity.
In some species, the posterior edges of the mantle may fuse to form two siphons that serve to take in and exude water. One of the functions of the mantle is to secrete the shell. This property has been commercially exploited to produce pearls. Gastropoda includes shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell Figure 4. Shells may be planospiral like a garden hose wound up , commonly seen in garden snails, or conispiral , like a spiral staircase , commonly seen in marine conches.
Figure 4. The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling Figure 5. Most gastropods bear a head with tentacles, eyes, and a style.
A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species. The mantle cavity encloses the ctenidia as well as a pair of nephridia. Figure 5. During embryonic development of gastropods, the visceral mass undergoes torsion, or counterclockwise rotation of anatomical features. As a result, the anus of the adult animal is located over the head. Torsion is an independent process from coiling of the shell.
Figure 6. In Chiton the ctenidia are present along the margin of the body excepting the head and anus, but in Patella the ctenidia are extended throughout the body. When the ctenidia remain restricted to a particular area of the body, it is called the merobranchiate type.
The merobranchiate type of ctenidia can be subdivided into the following types depending on the arrangement of leaflets. In Neomenia a tuft of filaments arises from the cloacal wall. This type of ctenidia consists of flattened gill filaments arranged in only single side of the ctenidial axis as observed in Pila, Triton.
When both of them are of same sizes. This is the characteristic of the Bivalves. Amongst Bivalves they become variously modified Fig. Nucula possesses short flat leaflets Fig. In some forms long filamentous leaflets are present. These filaments may be free as in Area Fig. In Unio Fig. In Poromya Fig. This type of ctenidium is characteristic of the Cephalopods.
Detailed structure is described in the biology of Sepia. In Doris see Fig. In Pterotrachea the mantle fold is absent and the filamentous branchial leaflets project freely and remain uncovered. In most cases the mantle may serve as respiratory organ. In Chaetoderma the gills are two in number and are symmetrically placed one on each side of the cloaca. Single distinct gill is present in Neomenia and in Proneomenia, the gills are nothing but a few folds on the cloacal wall.
In Aeolis, Aedida many highly vascular secondary gills Cerata are present on the dorsal surface of the body see Fig. The heart and the gills are intimately related because the main function of the gills is to aerate blood on its way to the heart.
The number of gills are directly proportional to the number of the auricles Fig. For example, when the gills are two in number, two auricles are present as encountered in Octopus and Loligo. In Chiton two auricles correspond to the two sets of the multiple gills, In Nautilus there are four gills and four auricles.
When the gill is unpaired, the heart has one auricle as seen in Opisthobranchs, Mesogastropods, Neogastropods, etc. Terrestrial habit leads to complete loss of gills and a variety of respiratory organs develop to suit the particular environment. In most Pulmonata the mantle cavity forms a pulmonary chamber, the inner surface of which is highly vascularised.
In Monotocardia Mesogastropoda the left nuchal lobe is better developed and forms a long respiratory siphon. These forms are exemplified by Pila. It possesses both ctenidium as well as pulmonary sac. The genus Siphonaria is furnished with a lung- cavity and a ctenidium. The circulatory system in Molluscs is quite well-developed. Almost all the members of the phylum excepting Scaphopoda which lacks a distinct heart have distinct heart which receives oxygenated blood from the respiratory organs and conveys it to the different parts of the body.
The respiratory pigment is haemocyanin, which contains both iron and copper. Haemoglobin is also present in some rare cases, e. The stomach further breaks down the food. The indigestible particles are taken to the intestine and excreted through anus.
Partially digested food goes to digestive gland for more intense digestion. Oxygen and nutrients are carried by the blood to all parts of the body. Excretion: The solid wastes are removed as feces through the anus.
The metabollic waste materials are removed by nephridia. Nephridia are invertebrate organs which have function similar to kidneys. Reproduction: Most of pelecypods are dioecioius, which means that individuals are either male or female. Most species simply just shed their eggs and sperms directly into the sea all at the same time. A free swimming larvae is developed after the sperm and egg fertilized. They also secretes a layer called "Mother of Pearl".
Most bivalves are sessile; however, some, such as scallops, can move around rapidly by flapping their shells when threatened.
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