file 2 KINGDOM ANIMALIA: PHYLUM ASCHELMINTHES AND PHYLUM ANNELIDA INTRODUCTION As per the presence or absence of the notochord, …

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KINGDOM ANIMALIA:

PHYLUM ASCHELMINTHES AND PHYLUM ANNELIDA

INTRODUCTION

As per the presence or absence of the notochord, the Kingdom Animalia can be broadly

classified as:

1. Non-Chordata: Animals that do not have the notochord.

2. Chordata: Animals that have the notochord persistent or at some phase of their

life.

The phyla Aschelminthes and Annelida represent the non-chordates. These have been

discussed in details in the following heads.

PHYLUM ASCHELMINTHES

GENERAL FEATURES

• These are mainly parasitic forms causing diseases in humans, animals, and plants.

There are few free-living forms as well.

• Generally called the roundworms because the cross-section of the body appears

round.

• The body has tapering ends on both the extremities.

• Their body is unsegmented

• Their embryo is triploblastic. The outer layer is ectoderm that forms the covering

of the body. The middle layer called mesoderm forms the muscles. The innermost

layer called the endoderm forms the alimentary canal.

• The body wall is made up of a tough and resistant layer called the cuticle, followed

by the syncytial epidermis (in which the nuclei are not separated by the cross-wall)

and then the muscular layer.

• The body cavity is pseudocoelom. The body cavity is not true as it is not surrounded

by the mesoderm on both the sides. It is filled with the fluid that provides the

hydroskeleton.

• The skeletal system, circulatory system, and respiratory system are absent.

• The exchange of gases occurs through the body surface.

DIGESTIVE SYSTEM:

Due to the presence of separate openings of mouth and anus, the digestive system is

regarded as complete. The pharynx is supplied with muscles. The mouth is covered with

six lips. They perform extracellular digestion. The digestive system does not show the

stomach. It is a long tubular structure.

EXCRETORY SYSTEM:

These are ammonotelic animals, that is they excrete ammonia. The renette cells are the

main excretory organs that are anteriorly present. They make the excretion via the

excretory pore, which also receives the excretory product from the excretory ducts. (Fig.1)

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NERVOUS SYSTEM:

The brain is represented by the circular ring present near the pharynx. There are four

nerve cord joining it, namely dorsal cord, ventral cord, and two lateral cords.

REPRODUCTION AND DEVELOPMENT:

These are dioecious animals. The males and females sex organs are present in different

organisms. They also show sexual dimorphism such that the males are shorter than the

females. Fertilization occurs internally. The indirect development involves larva formation.

There are the following three types of larva seen in different species:

a) Filariform larva, e.g., hookworm

b) Microfilaria larva, e.g., Wuchereria

c) Rhabditiform larva, e.g., Ascaris

Ascaris lumbricoides: Morphology

• It is a parasitic roundworm that appears slender, cylindrical with the tapering ends.

• It is an endoparasite in humans, pigs, cats, dogs, horses etc.

• It has sense organs for touch called the papillae, located near the lips. The sense

organs for the chemicals are called the amphids.

• It shows sexual dimorphism.

• It causes the disease cause ascariasis, that is more common in children. The infection

impairs the growth and also cause mental retardation.

Ascaris lumbricoides: LIFE-CYCLE

There is no secondary host for Ascaris. It completes its life-cycle in a single host and

produces rhabditiform larva that is capable of surviving in the soil. The infection of Ascaris

begins with the ingestion of the eggs through the ingestion of contaminated food with

faecal matter. The eggs reach the intestines and hatch there. These can survive the acidic

condition in the stomach due to the tough covering. The larvae enter the bloodstream by

crossing the mucosa of the intestine. They reach the lungs to perform aerobic respiration

essential for their growth. From the lungs they reach the pharynx via the trachea and are

swallowed back into the oesophagus from the pharynx. They mature in the intestine to

develop into adult males and females. The fertilization occurs in the intestine and some

eggs are released along with the faeces while some are retained in the intestine for the

next development round.

The infection causes diarrhoea, vomiting, impaired digestion, abdominal pain etc.

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PHYLUM ANNELIDA

GENERAL FEATURES:

• These animals have truly segmented body because the segments are visible

externally in the form of annuli and internally in the form of septation.

• The segmented body allows a greater degree of movement. Each segment has

almost all the parts of the body, so there is built-in mechanism of redundancy that

provides enhanced survival. Each segment is called the metamere.

• Their embryo is triploblastic. The outer layer is the ectoderm followed by the middle

layer of mesoderm and then the innermost endoderm.

• These have a true coelom. They mesoderm lines the body cavity from both the sides.

• These are protostome. The mouth develops before the development of anus.

• The coelom is filled with the fluid that serves as the hydroskeleton.

DIGESTIVE SYSTEM:

They have a complete digestive system. The first segment called the peristomium bears

the oral cavity. It is covered with the prostomium. There is a pharynx, gizzard (for

mastication of food), stomach and long intestines. The segment of intestine that shows

the fold is called the typhlosole. The anus is situated in the last segment.

EXCRETORY SYSTEM:

The nephridia are excretory organs (Fig 2). There are generally three types of nephridia

seen:

a) Septal nephridia that open into the intestine. These are enteronephric.

b) Pharyngeal nephridia that open into the pharynx. These are also enteronephric.

c) Integumentary nephridia that open to the outer space. These are exonephric.

Chloragogen cells are also excretory organs that absorb the excretory products from the

blood capillaries and coelomic fluid. They open into septal nephridia. They also store fats

and glycogen.

CIRCULATORY SYSTEM:

They have a closed circulatory system with blood flowing in the capillaries. The blood is

red due to haemoglobin present in the plasma.

NERVOUS SYSTEM:

The brain is present as a circular ring of nerves anteriorly. There is a ventral nerve cord

that runs along the length of the body. Various nerves arise from the ventral nerve cord

(Fig. 3)

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LOCOMOTION:

In terrestrial forms, the setae perform the locomotion that are situated deep in the body.

The muscles provide contractile ability as well. Parapodia are the locomotory organs in

aquatic forms, that also help in swimming.

REPRODUCTION:

These are generally monoecious animals. The male and female reproductive organs are

present in the same organism. The male organs mature first, so cross-fertilization occurs

generally. The sperms are stored in special organs called spermatheca. The clitellum

secretes mucus that forms the cocoon for the eggs. In monoecious forms, the development

in direct without larva formation. In dioecious forms, the development is indirect with the

formation of trochophore larva.

CLASSIFICATION OF PHYLUM ANNELIDA:

Class Polychaeta

• There are numerous setae present on the extension of body wall

called parapodia.

• These are marine and carnivorous forms.

• Clitellum is absent.

• These are dioecious Annelida.

• They form the trochophore larva.

• E.g., Nereis, Aphrodite etc.

Class

Oligochaeta

• They have a few setae that are locomotory organs.

• These are terrestrial as well as freshwater forms.

• Parapodia are absent.

• Clitellum is present that secretes cocoon for eggs.

• These are monoecious. Larval stage is not involved in

development.

• E.g., Pheretima (earthworm)

Class Hirudinea

• These are ectoparasites found in freshwater.

• Both the parapodia and setae are absent. Contraction of body

wall allows locomotion.

• Suckers are present anteriorly towards the ventral surface.

• These are dioecious.

• E.g., Hirudinaria

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