Phylum

 Phylum:  Platyhelminthes :-This  phylum  includes  flatworms.  These  are  acoelomates,  without  abody  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 organsin 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 Trematoda  :-  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 includes all tape worms. These are internal parasiteswith a complex life history. The life cycle involves two hosts.Their body characters are adaptations for parasitic life. Mouth and alimentary canal are absent. Food is absorbed through general body surface.The head is called the scolex. It has a ring of hooks and suckers for attach- ment to the host tissue. The body consists of several segments called Proglottids. (eg) sheep and cattle tape

worms.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.

Class  Cestoda :-  It  includes  all  tape  worms.  These  are  internal  parasiteswith a complex life history. The life cycle involves two hosts.Their  body  characters  are  adaptations  for  parasitic  life. Mouth  andalimentary canal are absent. Food is absorbed throughgeneral body surface.The head is called the scolex. It has a ring of hooks and suckers for attach-ment  to  the  host  tissue.  The  body  consists  of  several  segments  called Proglottids.  (eg)  sheep  and  cattle  tape  worms.

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. formed of ganglia

in the head region. The nerve cord is ventral in position.

For the first time head formation or cephalization happens. These are bi-sexual and hermophroditic. The larva is called the trochophore. Leech Nereis formed of  ganglia in the head region. The nerve cord is ventral in position. For the first time head formation or

Cephalization happens. These are bi- sexual and hermophroditic. The larva is called the  trochophore .This phylum includes three Classes, namely Polychaeta,  Oligochaetaand  Hirudinia. The polychaetes are marine worms. They have a distinct head. There arepairs of lateral projections called parapodia. The examples are

Nereis (ragworms), Arenicola(lugworm).Earthworms are included in the Class Oligochaeta. The Class: Hirudinia includes leeches. These are blood suckers and ectoparasites. They have well developed suckers for attachement at anterior and posterior ends.