Kingdom Animalia — Biology STD 11 Science — Question
Maharashtra BoardEnglish MediumSTD 11 ScienceBiologyKingdom Animalia5 Marks
Question
Animals whose body is covered by shell.
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Answer
Mollusca (Mollis: Soft) is the second largest phylum. Unique features of phylum Mollusca:
Habitat: They are aquatic or seen in marshy places. Few of them are terrestrial.
Forms: Molluscs are either free-living or sedentary.
Body plan: These are soft bodied and show tube within a tube type of body plan.
Body symmetry: Most of the Molluscs show bilateral symmetry but few are asymmetrical due to torsion (twisting).
Body division: Body consists of head, foot and visceral mass. Visceral mass is enclosed in thick, muscular fold of body wall called mantle. Mantle secretes a hard-calcareous shell, that may be external or internal or absent. Muscular foot is present ventrally.
Digestive system: Digestive system is well-developed and complete with anterior mouth and posterior anus. Buccal cavity has a rasping organ called radula (helps in feeding), which is provided with transverse rows of teeth.
Respiration: In aquatic forms, numerous feather-like gills called ctenidia, help in aquatic respiration. Terrestrial forms may show the presence of lungs.
Circulatory system: Circulatory system is of open type (except in Sepia, where it is of the closed type). Blood contains a copper containing blue coloured respiratory pigment called haemocyanin.
Excretion: Excretion occurs by kidney like structures, also called ‘Organ of Bojanus’.
Nervous system and sense organs; Nervous system has three rjahs of Mf hxese, interconnected by commissures and connectives.
Sense organs: Sense organs such as eyes for vision, tentacles for tactile sensation and osphradia for testing purity of water is present.
Sexual reproduction: Sexes are usually separate. Animals of this phylum are mostly oviparous and the development is direct or indirect.