Which of the following animals does not undergo metamorphosis? 1. Earthworm 2. Tunicate 3. Moth 4. Starfish

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Which of the following animals does not undergo metamorphosis?

1. Earthworm
2. Tunicate
3. Moth
4. Starfish

Answer:1. Earthworm

👉Phylum – Annelida

They may be aquatic (marine and fresh water) or

terrestrial; free-living, and sometimes parasitic.

They exhibit organ-system level of body

organisation and bilateral symmetry. They are

triploblastic, metamerically segmented and

coelomate animals. Their body surface is

distinctly marked out into segments or

metameres and, hence, the phylum name

Annelida (Latin, annulus : little ring) (Figure 4.11).

They possess longitudinal and circular muscles

which help in locomotion. Aquatic annelids like

Nereis possess lateral appendages, parapodia,

which help in swimming. A closed circulatory

system is present. Nephridia (sing. nephridium)

help in osmoregulation and excretion. Neural

system consists of paired ganglia (sing. ganglion)

connected by lateral nerves to a double ventral

nerve cord. Nereis, an aquatic form, is dioecious,

but earthworms and leeches are monoecious.

Reproduction is sexual.

Examples : Nereis, Pheretima (Earthworm) and

Hirudinaria (Blood sucking leech).

👉EARTHWORM

Earthworm is a reddish brown terrestrial invertebrate that inhabits the

upper layer of the moist soil. During day time, they live in burrows made

by boring and swallowing the soil. In the gardens, they can be traced by

their faecal deposits known as worm castings. The common Indian

earthworms are Pheretima and Lumbricus.

👉Morphology

Earthworms have long cylindrical body. The body is divided into more

than hundred short segments which are similar (metameres about

100-120 in number). The dorsal surface of the body is marked by a dark

median mid dorsal line (dorsal blood vessel) along the longitudinal axis of

the body. The ventral surface is distinguished by the presence of genital

openings (pores). Anterior end consists of the mouth and the prostomium,

a lobe which serves as a covering for the mouth and as a wedge to force

open cracks in the soil into which the earthworm may crawl. The prostomium

is sensory in function. The first body segment is called the peristomium

(buccal segment) which contains the mouth. In a mature worm, segments

14-16 are covered by a prominent dark band of glandular tissue called

clitellum. Thus the body is divisible into three prominent regions –

preclitellar, clitellar and postclitellar segments (Figure 7.9).

Four pairs of spermathecal apertures are situated on the ventro-lateral

sides of the intersegmental grooves, i.e., 5th -9th segments. A single female

genital pore is present in the mid-ventral line of 14th segment. A pair of

male genital pores are present on the ventro-lateral sides of the 18th

segment. Numerous minute pores called nephridiopores open on the

surface of the body. In each body segment, except the first, last and

clitellum, there are rows of S-shaped setae, embedded in the epidermal

pits in the middle of each segment. Setae can be extended or retracted.

Their principal role is in locomotion.

👉Anatomy

👉The body wall of the earthworm is covered externally by a thin non-cellular

cuticle below which is the epidermis, two muscle layers (circular and

longitudinal) and an innermost coelomic epithelium. The epidermis is made

up of a single layer of columnar epithelial cells

which contain secretory gland cells.

The alimentary canal is a straight tube and

runs between first to last segment of the body.

(Figure 7.10). A terminal mouth opens into the

buccal cavity (1-3 segments) which leads into

muscular pharynx. A small narrow tube,

oesophagus (5-7 segments), continues into a

muscular gizzard (8-9 segments). It helps in

grinding the soil particles and decaying leaves,

etc. The stomach extends from 9-14 segments.

The food of the earthworm is decaying leaves and

organic matter mixed with soil. Calciferous

glands, present in the stomach, neutralise the

humic acid present in humus. Intestine starts

from the 15th segment onwards and continues

till the last segment. A pair of short and conical

intestinal caecae project from the intestine on the

26th segment. The characteristic feature of the

intestine after 26th segment except the last

23rd-25th segments is the presence of internal

median fold of dorsal wall called typhlosole. This

increases the effective area of absorption in the

intestine. The alimentary canal opens to the

exterior by a small rounded aperture called anus.

The ingested organic rich soil passes through the

digestive tract where digestive enzymes

breakdown complex food into smaller absorbable

units. These simpler molecules are absorbed

through intestinal membranes and are utilised.

👉Pheretima exhibits a closed type of

blood vascular system, consisting of blood

vessels, capillaries and heart. (Figure 7.11). Due

to closed circulatory system, blood is confined

to the heart and blood vessels. Contractions

keep blood circulating in one direction. Smaller

blood vessels supply the gut, nerve cord, and

the body wall. Blood glands are present on the

4th, 5th and 6th segments. They produce blood

cells and haemoglobin which is dissolved in

blood plasma. Blood cells are phagocytic in

nature. Earthworms lack specialised breathing

devices. Respiratory exchange occurs through

moist body surface into their blood stream.

👉The excretory organs occur as segmentally

arranged coiled tubules called nephridia

(sing.: nephridium). They are of three types:

(i) septal nephridia, present on both the sides of

intersegmental septa of segment 15 to the last

that open into intestine, (ii) integumentary

nephridia, attached to lining of the body wall of

segment 3 to the last that open on the body

surface and (iii) pharyngeal nephridia, present

as three paired tufts in the 4th, 5th and 6th

segments (Figure 7.12). These different types of

nephridia are basically similar in structure.

Nephridia regulate the volume and composition

of the body fluids. A nephridium starts out as a

funnel that collects excess fluid from coelomic

chamber. The funnel connects with a tubular part

of the nephridium which delivers the wastes

through a pore to the surface in the body wall

into the digestive tube.

👉Nervous system is basically represented by

ganglia arranged segmentwise on the ventral

paired nerve cord. The nerve cord in the anterior

region (3rd and 4th segments) bifurcates, laterally

encircling the pharynx and joins the cerebral

ganglia dorsally to form a nerve ring. The cerebral

ganglia alongwith other nerves in the ring

integrate sensory input as well as command

muscular responses of the body.

👉Sensory system does not have eyes but does

possess light and touch sensitive organs (receptor

cells) to distinguish the light intensities and to feel

the vibrations in the ground. Worms have

specialised chemoreceptors (taste receptors) which

react to chemical stimuli. These sense organs are

located on the anterior part of the worm.

Earthworm is hermaphrodite (bisexual), i.e.,

testes and ovaries are present in the same

individual (Figure 7.13). There are two pairs of

testes present in the 10th and 11th segments.

Their vasa deferentia run up to the 18th segment

where they join the prostatic duct. Two pairs of

accessory glands are present one pair each in

the 17th and 19th segments. The common prostate

and spermatic duct (vasa deferentia) opens to

the exterior by a pair of male genital pores on

the ventro-lateral side of the 18th segment. Four

pairs of spermathecae are located in 6th-9th

segments (one pair in each segment). They receive

and store spermatozoa during copulation. One

pair of ovaries is attached at the inter-segmental

septum of the 12th and 13th segments. Ovarian

funnels are present beneath the ovaries which

continue into oviduct, join together and open on

the ventral side as a single median female genital

pore on the 14th segment.

👉A mutual exchange of sperm occurs between

two worms during mating. One worm has to find

another worm and they mate juxtaposing

opposite gonadal openings exchanging packets

of sperms called spermatophores. Mature sperm

and egg cells and nutritive fluid are deposited in

cocoons produced by the gland cells of clitellum.

Fertilisation and development occur within the

cocoons which are deposited in soil. The ova

(eggs) are fertilised by the sperm cells within the

cocoon which then slips off the worm and is

deposited in or on the soil. The cocoon holds the

worm embryos. After about 3 weeks, each cocoon

produces two to twenty baby worms with an

average of four. Development of earthworms is

direct, i.e., there is no larva formed.

👉Earthworms are known as ‘friends of farmers’ because they make

burrows in the soil and make it porous which helps in respiration and

penetration of the developing plant roots. The process of increasing fertility

of soil by the earthworms is called vermicomposting. They are also used

as bait in game fishing.