Protochordata (Acrania)

Protochordata (Acrania) The protochoradates are considered as the fore runners of vertebrata The classification of the protochordates is based on the nature of the noto- chord.

Sub phylum : Hemichordata .These are exclusively marine organisms. They are solitary or colonial forms. They mostly remain as tubiculous forms. The body is soft, vermiform, unsegmented,bilaterally symmetrical and triploblastic. The body is divisible into three distinct regions namely proboscis, collar and trunk. The body wall is composed of single layer of epidermal cells. The dermis is absent. They have no endoskeleton. A projection from pharynx, projecting inside the proboscis  may be considered as notochord. They have a spacious coelom lined by coelomic epithelium. The alimentary canal is a straight tube running between mouth and anus. They are ciliary feeders. Sexes are separate. Examples :  Balanoglossus ,  Saccoglossus .

Protochordata (Acrania)

The protochoradates are considered as the fore runners of vertebrata

The classification of the protochordates is based on the nature of the noto- chord.

Sub phylum : Hemichordata . These are exclusively marine organisms. They are solitary or colonial forms. They mostly remain as tubiculous forms. The body is soft, vermiform, unsegmented,bilaterally symmetrical and triploblastic. The body is divisible into three distinct regions namely proboscis, collar and trunk. The body wall is composed of single layer of epidermal cells. The dermis is absent. They have no endoskeleton. A projection from pharynx, projecting inside the proboscis  may be considered as notochord. They have a spacious coelom lined by coelomic epithelium. The alimentary canal is a straight tube running between mouth and anus. They are ciliary feeders. Sexes are separate.

Examples : Balanoglossus , Saccoglossus .

Sub phylum : Cephalochordata .

Cephalochordates are small fish like marine chordates. The persistent notochord extends forward beyond the brain . Hence these are called cephalochordates. The epidermis is single layered. Paired fins are absent. Muscles, nephridia and gonads are segmentally arranged. The pharynx is large with numerous gills. It is a filter feeder. Example :  Amphioxus.

Diversity of Chordates

The chordates exhibit an astonishing diversity in form, physiology

And habits. The number of chordate species is limited. About 49,000 species are on record which are only half of the living species of molluscs and less than one tenth of arthropods. Despite their modest number of species, the chor- dates make remarkable contribution to the bio-mass of the earth. Nearly all of them are medium to large in size. The vertebrates in particular are consider-

ably larger and many of them are among the largest of living animals. The gigantic  blue whale which is 35 meters long and 120 tons in weight is the biggest known animal. The smallest vertebrate ,

philippine goby is a fish, measuring only 10 mm in length. The chordates are able to occupy various kinds of habitats. They have adapted themselves to more modes of existence than any other group. They are found in the sea, in freshwater, in the air and

on all parts of land from the poles to the equator.

General Characters :

The three distinctive characteristics of the chordates are the presence

of  notochord, dorsal tubular nerve cord and  pharyngeal gill slits .

1. Notochord : During the embryonic development of a chordate there appears a sup- porting rod called the  notochord . It lies dorsal to the alimentary canal and ventral to the nerve cord. In some chordates this structure persists throughout life. In others it is partially or completely replaced by a ‘ backbone . It is made up of separate  bony elements or  vertebrae. Structurally it is com- posed of large number of specialized  vacuolated  cells. It is surrounded by fibrous and  elastic sheath . The stiffness of the notochord is due to the tur- gidity of fluid-filled cells and surrounding connective tissue sheath.

2. Dorsal tubular nerve cord  The nerve cord lies just above the notochord and remains entirely out- side the coelom. It is a tubular structure having a small hollow canal running from one end to the other. The dorsal hollow nerve cord persist throughout the adult life of almost all chordates.

3. Gill slits or Pharyngeal clefts These are paired lateral clefts leading from the pharynx to the exte-

rior. They are present throughout life in fishes and a few tailed amphibians. In amphibians, like frogs and toads it is found only in the larval stages. In higher vertebrates (reptiles, birds and mammals) they are embryonic and non-func- tional.

4. Ventral heart The heart is chambered. It is located ventral to the alimentary canal.

5. Closed blood vascular system

In chordates, the blood passes through a continuous system of tubes

namely arteries, capillaries and veins.

6. Hepatic portal system In chordates, the food laden blood from the digestive tract passes

through the capillary net work in the liver, before reaching the heart. Thus the veins originating from the digestive tract as capillaries and ending in the liver again as capillaries constitute the hepatic portal system.

Classification.

The Phylum Chordata is classified into four sub phyla:

Sub phylum 1. Hemichordata,

Sub phylum 2. Cephalochordata

Sub phylum 3. Urochordata

Sub phylum 4. Vertebrata.

First three sub phyla are collectively known as

Protochordates . Since the members of these sub phyla do not have a cranium or skull they are also referred to as  Acrania.

Phylum Echinodermata :-

Phylum Echinodermata :- These are marine organisms. While the adults are radially symmetrical the larvae remain bilaterally symmetrical. The mouth is on the lower surface. They have a  water vascular system with  tube feet. eg. star fishes, brittle stars, sea urchins and sea-cucumbers. Phylum Chordata This phylum derives its name from one of the common characteristics of this group namely the  notochord(Gr. noton, back + L. chorda, cord). The

animals belonging to all other phyla of the Animal Kingdom are often termed ‘the non -chordates’ or ‘the invertebrates’ since they have neither notochord nor backbone in their body. The backboned animals (vertebrates), together with a few closely re-lated animals which do not possess a backbone, are included in this phylum. Most of the living chordates are familiar vertebrate animals. The chordates are of primary interest because human beings are members of this group.

Diversity of Chordates

The chordates exhibit an astonishing diversity in form, physiology and habits. The number of chordate species is limited. About 49,000 species are on record which are only half of the living species of molluscs and less than one tenth of arthropods. Despite their modest number of species, the chor- dates make remarkable contribution to the bio-mass of the earth. Nearly all of them are medium to large in size. The vertebrates in particular are consider- ably larger and many of them are among the largest of living animals. The gigantic blue whale which is 35 meters long and 120 tons in weight is the biggest known animal. The smallest vertebrate ,  philippine goby is a fish,

measuring only 10 mm in length. The chordates are able to occupy various kinds of habitats. They have adapted themselves to more modes of existence than any other group. They are found in the sea, in freshwater, in the air and on all parts of land from the poles to the equator.

General Characters : The three distinctive characteristics of the chordates are the presence of  notochord, dorsal tubular nerve cord and pharyngeal gill slits.

Phylum

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 Arthropoda characters. Hence this may be considered as a  connecting link between the two groups.

Class Crustacea :- The examples for this class are  prawns ,

Crabs and lobsters . The dorsal body surface is covered by a sheild like carapace .

Class Myriapoda:- It includes centepedes and  millipedes. These organ-isms have a distinct head and simple eyes. The centepedes have a pair ofpoison claws. The body consists of numerous segments, bearing pairs of legs.

Class Insecta:- It comprises the common insects. The body is divided into head thorax and abdomen.In several insects, the adults have two pairs of wings on the thorax. Respiration happens through the tracheal system.

Class Arachnida :- It includes scorpions, spiders, ticksand mites. The body is divided into  cephalothorax and abdomen. There are four pairs oflegs attached to the cephalothorax.

Class Insecta:- It comprises the common insects. The body is divided into head thorax and abdomen.In several insects, the adults have two pairs of wings on the thorax. Respiration happens through the tracheal system.

Class Arachnida:- It includes scorpions, spiders, ticks and mites. The body is divided into cephalothorax  and abdomen. There arefour pairs of legs attached to the cephalothorax.

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, oysters.

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 tentacles and eyes. (eg) snails, slugs, and limpets.

Class Cephalopoda:- These are mostly marine. They are adapted for

swimming. The foot is modified into eight to ten long tentacles in the head region. The shell is either internal or absent. (eg) Octopus, Loligo, Sepia.

Phylum Echinodermata:- These are marine organisms. While the adults are radially symmetrical the larvae remain bilaterally symmetrical. The mouth is on the lower surface. They have a

water vascular system with tube feet. eg. star fishes, brittle stars,

sea urchins and  sea-cucumbers .

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.