Zoology optional paper 2019









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Status of Hemichordata

Hemichordata status                           Introduction Hemichordata was till recently treated as the sub phylum of the phylum Chordate but is now regarded to be the independent phylum of invertebrates very close to the phylum Echinodermata. The peculiar characteristics of the animals belonging to Hemichordata are as follows, Enterocoelus coelom Pharyngeal gill slits Buccal diverticulum (earlier considered as notochord) Vermiform body divisible into three regions – proboscis, collar, trunk The phylum hemichordate includes a small group of soft, primitive and marine chordates called as acron worms or tongue worms. Most familiar genus of this phylum is Balanoglossus which belongs to the class Enteropneusta. Other closely related genera are Saccoglossus, Glossobalanus etc. The class Enteropneusta was established by Gegenbaur in 1870. Since then due to the peculiar anatomical organisation and embryology, Hemichordata have...
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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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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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