Medical brain notes 33

  Methods of grouping animals   There are several ways of grouping animals. In all these methods the basic Taxon remains without any change. However the taxa are rearranged in different groups. All these groupings are mostly provided for the convenience in identifying similar taxa.   I.  One of the earliest method of grouping the animals could be dividing the Animal kingdom into two assemblages called  Invertebrata  and  Vertebrata . This scheme was provided initially by Aristotle. This scheme does not have a place for the Prochordates.   II.  Animals can also be grouped as  single celled  and  multicellular . The single celled organisms are called the  Protozoans . The multicellular could be called the Metazoans . In this arrangement among the metazoans the unique nature of the sponges in not having a tissue grade of body constuction is not mentioned.   III.  In yet another method the animals are grouped under f...

  Phylum : Protozoa,  Porifera and  Coelenterata or Cnidaria   Phylum : Protozoa   This phylum includes a great diversity of small, microscopic organ-isms. These are  single celled  eukaryotes. Their locomotion happens using pseudopodia, cilia or flagella.   The nutrition is either autotrophic or heterotrophic. They reproduce either asexually or by sexual methods. Ex : Amoeba, Paramoecium, Plasmodium.   Phylum : Porifera .   These are multicellular, aquatic organisms. They have a  cellular grade  of construction without the occurrence of tissues. The sponges belonging to this phylum are characterised by the presence of a  canal system  in their body. The body wall contains spicules. They can reproduce both by asexual and sexual methods. Ex : Sponges. Phylum : Coelenterata or Cnidaria All coelenterates are aquatic animals. They are mostly marine. The body is radially symmetrical. The body wall is of two layers of ...

  Phylum: Platyhelminthes  :-   This phylum includes flatworms. These are acoelomates, without a body cavity called  coelom . The alimentary canal is either absent or very simple. Excretion and osmoregulation occur through  flame cells .These worms are mostly hermophrodites, having both male and female reproductive organs in a single individual. Most of the members are parasites. It is divided into three classes, namely  Turbellaria ,  Trematoda  and  Cestoda . Class Turbellaria  :- These are free living aquatic flatworms. The Planaria of   this class shows characteristic regeneration.   Class Trematod a :- These are flukes living as parasites inside a host (en-doparasites). A protective  cuticle  covers the outer surface of the body. Flukes have suckers  for attachment to the host tissues. The examples are  Fasciola  (liver fluke),  Schistosoma  (blood fluke). Class Cestoda  :- It includ...

  Phylum : Nematoda  :-   These are the popular round worms. The body is narrow and pointed at both the ends. There are no body segments. The body is covered by a thin cuticle. The body cavity is considered as a pseudocoelom. The alimentary canal is a straight tube. They reproduce sexually and the sexes are seperate. There are several free living soil nematodes. Others are parasites. (eg)  Ascaris lumbricoides . In subsequent Phyla the animals show following general characters   1.     There is a coelom within the mesoderm. Hence these are called as coelomates.   2.     The body consists of a series of compartments. This phenomenon is called as metameric segmentation. They have a circulatory system pro-viding internal transport.   Phylum: Annelida :-   These are worm like animals. The body segments are rings externally. Internally the segments are seperated by  septa . Externally the body is protected by a cuticl...

  Phylum : Arthropoda  :-   These are the most successful group of animals. They outnumber all other animals in population strength. The body is segmented. It is covered by a hard exoskeleton made of  chitin . During growth the exoskeleton is shed (moulting of ecdysis). The legs or paired appendages are jointed. The head region has a pair of prominent compound eyes. Each compound eye is made up of several photoreceptor sub units called Ommatidia.   They have an open circulatory system without vessels. The body cavity is filled with a fluid called  haemolymph . Such body cavity is known as haemocoel. These are unisexual, exhibiting  sexual dimorphism . The young forms produced are invariably called the  larvae . The larvae undergo  metamorphosis  and develop into adults.   This Phylum comprises five Classes,  Class Onychophora : It includes small worm like  Peripatus .  Peripatus  shows Annelidan and Arthropo...

  Phylum Mollusca  :-   It is a very successful and diverse group of animals.   Considered to be the second largest group of animals with regard to species number. These are soft bodied animals without segmentation. The body is divided into  head ,  muscular foot  and  visceral mass . The body is covered by a  mantle  and a  shel l.   Respiration happens through gills (ctinidia) in the mantle cavity. The most common larva is a  trochophore  larva.   There are seven classes of which three are more prominent.   Class Pelecypoda  or  Bivalvia  :- These are aquatic molluscs having bivalves.   They burrow in mud and sand . The body is laterally compressed . (eg)  mussels ,  clams ,  oyster s.   Class Gastropoda  :- These are either aquatic or terrestrial molluscs. They   posses a spiral shell.   The foot is large and flat. They have well developed head with t...