EXPERIMENT Objective. To study the external morphology of earthworm, cockroach and frog through models.

 

                                 EXPERIMENT 

Objective. To study the external morphology of earthworm, cockroach and frog through models.

REQUIREMENTS

Models of Earthworm, Cockroach and Frog, Laboratory manual practical record book, pencil, pen, eraser etc.

PROCEDURE

Observe the model carefully and note down its morphological features. Draw it, on practical record file and write down the comments.

1. EARTHWORM (Pheretima posthuma)

External Morphology

It has an elongated, narrow and cylindrical body with both ends bluntly pointed. The dorsal surface of the body in darker is colour with a darker mid dorsal line. The ventral surface is marked by genital apertures and papillae located in anterior region of body.

Segmentation. The body is divided into about 100 to 200, small ring like segments (metameres) by distinct annular grooves or furrows in the body wall.

Clitellum. About two centimetres behind the front end of the body, there is present a thick collar like

glandular thickening of body wall called clitellum (cingulum). It occurs around the segments 14, 15 and 16. Due to clitellum, the body is divisible into preclitellar, clitellar and post clitellar regions. In preclitellar region, the first segment, called peristomium, bears a terminal month, and a dorsal fleshy, lobe like process, over hanging the month like a hood called prostomium.

Setae. Each segment, except the first and the last, bears an equatorial annular row of about 80 to 120

minute, S-shaped setae (chaetae). Setae assist in locomotion.

Apertures. Following apertures occurs upon the body surface.

(i) Mouth. It is transversely crescentic and terminal aperture at the enterior end. The protostomium dorsally over hangs upon it like a hood.

(ii) Anus. It is a small and vertical slit-like aperture at the end of last, anal segment.

(iii) Dorsal pores. Starting behind from the inter segmental groove, between segments 12 and 13 upto the last but one groove, there is a single and mid-dorsal minute pore in each groove. These are called dorsal pores and are meant for oozing out coelomic fluid to keep the body wall moist for respiration.

(iv) Nephridiopores. These are about 200 to 250 minute pores of nephridia scattered upon the surface of each segment except the first six.

(v) Spermathecal pores. In each of the four inter segmental grooves between segments 5/6, 6/7,7/8 and 8/9, There is a pair of ventro-lateral elliptical spermathecal pores.

(vi) Female genital pore. A single minute female genital pore is located in the mid-ventral line of 14th

segment.

(vii) Male genital pores. These are a pair of crescentic apertures located ventrolaterally upon 18th segment.

Genital papillae. Just behind the clitellum each of the 17th and 19th segments bears a pair of small and conical ventro-lateral genital or copulatory papillae which are the most prominent structure upon ventral side of the body. These papillae help in copulation.

2. COCKROACH (Periplaneta americana)

External morphology

Cockroach is comparatively large sized insect with somewhat dorso-ventrally flattened body. Sexes are separate. Males are comparatively smaller and more flattened.Male Cockroach A male cockroach shows following features:

1.Physical appearance. It is dark brown in colour. The body is elongated bilaterally symmetrical and

dorsiventrally flattened. It is about 3 to 4 cm in length and 0.8 to 1.0 cm in width.

2. Exoskeleton. The body is covered by a chitinous cuticle. Exoskeleton is in the form of numerous plates or sclerites jointed by sutures. It is impervious to water and provides protection to the soft body parts.

3. Body. The body is distinguished into head,

thorax and abdomen. Each part is further

divided into many segments.

(i) Head. It is pear shaped, antero- posteriorly flattened and hanging downwards. It consists of six fused segments. It is attached to the thorax by a short and narrow neck called cervisum. The head bears a pair of long jointed antenae, a pair of compound eyes and a set of bitting and chewing mouth parts. The mouth parts consists of:

(a) a labrum (upper lip)

(b) paired mandibles (jaws)

(c) two pairs of maxillae (1st pair is present beneath the mandibles and 2nd pair forms the lower lip) and

(d) hypopharynx (acts like tongue).

(ii) Thorax. It consists of three segments prosoma (prothorax), mesosoma (mesothorax) and metasoma,(metathorax). The exoskeleton of each thoracic segment consists of a dorsal plate, the tergum and ventral sternum. The tergum of the prosoma (prothorax) is largest and projects forward overlapping the neck. It is called pronotum. Thorax bears legs and wings.

(a) Legs. Three pairs of jointed legs are present on the thorax. Each legs is five segmented made up of a basal flat coxa, small trochanter, long femur, slender spiny tibia and tursus or foot.

(b) Wings. There are two pairs of wings. The first pair is attached to mesothorax and second pair to

metathorax. The first pair of wings is leathery, thick, brown and opaque. They are called elytra. They

provide protection to the body. The second pair of wings lies underneath the wings of first pair and are used in flight.

(iii) Abdomen. It is made up of 10 segments. Each segment consists of a dorsal terguma and a ventral sternum, jointed by pleuron. In male, the 8th tergum is overlapped by the 7th tergum. The 10th tergum is bilobed and below it, is a pair of filaments anal cerci. Ventrally the first sternum is rudimentary. In male, 9th sternum is visible. It bears a pair of anal styles. The genital aperture is found between the 9th and 10th sterna surrounded by a set of processes called gonapopyses. Beneath the 10th tergum at the posterior end of abdomen is found anus.

Spiracles. There are slit like openings present on the sides of the body segments. The first two pairs of spiracles are present on mesothorax and metathorax and rest 8 on 1st to 8th abdominal segments. Spiracles are the pores meant for exchange of gases.

Female Cockroach

A female cockroach also possesses the same features except of the following differences:

1.In female cockroach, the hind wings reach upto the tip of the abdomen.

2.In female, the sclerites of 8th and 9th segments are overlapped by the corresponding sclerites of the 7th segment. The 7th sternum is boat-shaped. It together with 8th and 9th sterna forms the brood pouch or the genital pouch.

3.The female gonopore is also situated in 9th sternum and is surrounded by gonapophyses which serve as ovipositor.

Differences between male and female cockroach

Male Cockroach

1.Abdomen comparatively long and narrow.

Abdomen short and broad.

2.Wings reach beyond the tip of abdomen.

3.7th segment covers only the 8th segment

of the abdomen.

4.All the nine abdominal segments are Only the first seven abdominal segments visible.

5.There is no brood pouch.

6.9th abdominal segment bears a pair of anal Anal styles 

Female Cockroach

1. Abdominal short and broad. 

2.Hind wings reach upto the tip of abdomen.

3.7th segment of the abdomen covers both

8th and 9th segments.

4.Only the first seven abdominal segment are visible. 

5. 7th . 8th and 9th segments form a brood pouch. 

6.Anal style absent. 

3. FROG (Rana tigrina)

External Morphology

Frog is bilaterally symmetrical. The body is broad and dorsoventrally flattened. It is covered with loosely attached skin which is mottled green on the dorsal side and pale yellow on the ventral side.

A The body is divisible into head and trunk.

Head. The head is flattened and somewhat triangular. It is prolonged into a snout. It bears a wide semi-circular mouth at its anterior extremity. At the top of anterior end, the head bear to small apertures called nostrils or external nares. A little behind the nostrils are present two large eyes, situated along the sides. The eyes are spherical and protruded laterally. Each eye has three eye lids. The third eye lid is transparent and is movable called nictitating membrane. A little below and behind each eye, is a black patch, in the centre of which is a circular area called tympanic membrane or tympanum or ear drum. The floor of the head is

occupied by a soft throat. In male frog, there is a

pair of blue coloured wrinkled patches of skin, one

on either side of the posterior part of the throat. They are called vocal sacs.

 Trunk. The head passes into trunk without

any line of demarcation. The trunk is broad and flat-

tened. At its posterior end lies a circular aperture

called the cloacal aperture. In the sitting posture

of the frog, there formed a hump on the mid-dorsal

surface of its trunk. There are two pairs of limbs,

attached to the trunk called fore limbs and hind

limbs. The fore limbs are shorter than the hind limbs.

Each fore limb consists of an upper arm, fore-arm and hand. The hand consists of wrist, palm and fore

fingers or digits. During breeding season, the male frog develops a swollen nuptial pad on the inner side of the first finger of hand. Each hind limb is Z-shaped and consists of three parts thigh, shank and foot. The foot consists of ankle, instep and five toes or digits. In between toes, there is a membranous fold of skin called web.