UPSC MAINS 2018 DAF

DAF for mains 2018 has been out
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CANAL SYSTEM IN Porifera

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Canal system in Porifera (sponges) Body of all sponges is the perforated by large number of apertures through which water enters Inside body and flows through a system of criss-crossing canals collectively forming the canal system which is a characteristic feature of poriferans. Following types of canal systems are found in sponges: Ascon type, with flagellated spongocoel Sycon type, with flagellated radial canals Leucon type, with flagellated chambers Rhagon type, with conical shape and broad base ASCON TYPE This is the simplest type of canal system and is found in  Leucosolenia  and other homocoela. Ostia are present on the surface of body and lead directly into the spongocoel, which is lined by flagellated choanocyte cells. Spongocoel opens to the outside through a narrow circular opening, the  osculum  located at the distal free end of the sponge body. Water enters through ostia into spongocoel and goes out of body through the  osculum . SYCON TYPE This type of ca
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Parasitic adaptation of Taenia

Taenia Parasitic adaptations Tapeworms live as internal parasites and they have to make several adjustments for the same. The following are few of the adaptations made by these tapeworms to make their life easy in their hosts. Presence of enzymatic resistant covering A hard covering called as tegument covers the body of these tape worms. Tegument is enzyme resistant and it protects all the internal organs from the digestive actions of the alkaline fluids of the host. On the other hand tegument is permeable to water and other nutrients. Absence of locomotory organelles This parasite lacks organs for locomotion. As there is no need for any locomotory organs both the adult and larval forms lack cilia or flagella. Osmotic pressure The osmotic pressure inside the body is higher than that of the surrounding host tissue or fluid. This helps the parasite to reside conveniently in the host body. Presence of hooks and suckers for attachment For the attachment purpose a scolex
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Retrogressive Metamorphosis

Retrogressive Metamorphosis in Urochordates Metamorphosis is a change from the juvenile to adult stage in which larval stage is quite different from the adult stage. In retrogressive metamorphosis the larva possesses advanced characters which are lost during the development and the adult is either sedentary or degenerated with primitive characters. Urochordate adults, being sedentary show degenerative characters while the free swimming tadpole larva shows advanced chordate characters which are lost during metamorphosis. Parasitic crustaceans, like Sacculina and copepod parasites and stylopids and scale insects (Insecta) also show retrogressive metamorphosis. Retrogressive Metamorphosis in Herdmania The tadpole larva of Herdmania is only 1-2 mm long when it hatches out of the egg. It does not feed and hence has only 3 hours of survival during which it has to swim about in search of a suitable substratum for attachment. The larva needs advanced features for its free swimming existence,
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