9.3.5 Phylum Mollusca (Soft Bodied Animals) (L. Mollis, soft).
The term mollusk was introduced by Kontson in 1650 (about 50,000 species are recorded).
Habitat and habits:
Mostly free living, some are attached (sessile) found in freshwater, marine, moist soil, and mountain rocks.
Body:
Triploblastic, soft-bodied animals. They are mostly haemocoelomate, coelom is divided into blood spaces. The body contains the head, visceral region and
Mantle:
The whole body is covered in an envelope of gland foot.
Shell:
The mantle in many cases secretes a calcareous shell, which may be internal (cuttlefish) or external (snail), some without shell (octopus). tentes. tissues known as the Blood circulatory system is of
Systems:
most systems are well-developed (such as digestive, circulatory, respiratory, and reproductive systems). Respire through gills or an open type (except, class cephalopods). Radula: Tongue-like structure provided with horny teeth. Nephridia are their excretory organs (may be one, two, or six).
The nervous system consists of a collection of ganglia in the head region forming a ganglionic mass, which is connected with the ganglia of the foot and visceral I mass. Mostly Unisexual few are bisexual. Indirect development (trochophore larva). First invertebrate in which both endo and exoskeletons were originated.
Evolutionary Adaptations in Molluscs:
Mollusks have a complete digestive system, in the mouth cavity of many mollusks have rasping tongues (Radula) many horny teeth, and help in scraping the flesh of animals and the wood of ships. (e.g., Toredo)
Respiration occurs through gills, in many snails the mantle cavity is converted by a lung. Most molluscs possess an open blood circulatory system, consisting of the heart and blood spaces (sinuses). Excretion takes place through metanephridia, which opens into the pericardial cavity, the nervous system consists of three pairs of interconnected ganglia in the head, foot, and viscera. The brain of an octopus is large and complex enclosed in a shell and has got great ability to learn.
Economic importance of Molluscs:
They are both useful and harmful.
Useful Mollusks:
Some oysters make precious pearls e.g., pearl oysters. Used in the button industry (such as the shell of freshwater muss Used in road building (oyster shells are mixed with tar). Used for making ornaments. Source of food in many countries (such as oyster, mussel, clam). Oysters are used as for decoration.
Harmful Molluscs:
They cause injuries while working e.g., slugs. They destroy the woods of ships e.g. Toredo a shipworm. 3. They destroy plants by cutting up their roots and stems. 9.3.6 Phylum Annelida (Segmented Worms)(Latin, annulus; a little ring, Greek: eidos means form). Habit and habitat: Free-living, found on soil (earthworm) fresh water, and marine (Nereis), some are ectoparasites (leech).
Body:
Triploblastic, metamerically segmented (Metamerism: the body is divided into segments both internally and externally by a transverse partition called septa (septum is a membrane) between segments. Possess true coelom, divided into compartments by septa containing coelomic fluid also acts as a hydrostatic skeleton (i.e., becomes hard by fluid pressure).
Digestive system:
It is simple in free-living annelids while less developed in parasitic forms, consisting of a straight alimentary canal with a mouth and anus at opposite ends. Circulatory system: It consists of a closed blood circulatory system. (Note: first animal group, in which Closed Blood vascular system originated).
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