Morphology of Earthworm, Cockroach and Frog

Introduction

Morphology is a branch of biology that deals with the study of the form and the structure of organisms and their specific structural features. This includes many aspects like shape, structure, colour pattern, size and appearance and as well as, it includes internal parts like bones and organs. This article explains the external morphology of some creatures like the earthworm, frog and cockroach.

Morphology of Earthworm

Earthworm is a reddish-brown terrestrial invertebrate that inhabits the upper layer of the moist soil. During the day, 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.

Earthworm is nocturnal i.e., active during night. Body is cylindrical, bilaterally symmetrical, elongated with metameric segmentation. Number of segments is about 100-120 which are similar and length is about 150 mm.

Anterior end consists of the mouth and the prostomium, a lobe that serves as a covering for the mouth and as an earthworm may crawl. The first body segment is called the peristomium (buccal segment) which contains the mouth.

Earthworm is brown or clay-coloured because of pigment porphyrin which protects the body from the injurious effects of bright light.

Three regions in the body of earthworm are: Preclitellar (1-13 segment), Clitellar region (14, 15, 16 segment) and Postclitellar (17-last segment).

Spermathecal pores are located ventrolaterally in the intersegmental grooves of 5/6, 6/7, 7/8 and 8/9 (4 pairs). Female genital pore is on the 14th segment on the midventral line and one pair of male genital pores are present on the 18th segment on the ventrolateral side. Genital papillae are present on segments 17th and 19th.

The Dorsal surface is marked by a dark medium mid-dorsal line because of the dorsal blood vessel.

Along the mid-dorsal line in intersegmental grooves are located dorsal pores from 12/13 to the last but one segment. Body wall consists of cuticle (thin and non-cellular), epidermis (single layer of columnar epithelial cells), muscular layer and parietal peritoneum.

Epidermis consists of tall, columnar cells of four types – Supporting cells, Glandular cells, Basal cells and sensory cells. 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.

Setae and contraction of muscles help in locomotion.

The body cavity of earthworm is true coelom and is filled with milky white alkaline coelomic fluid.

Chloragogen cells are present in coelomic fluid. These are small, star-shaped, yellow cells concerned with storage of reserve food, deamination of proteins, formation of urea and also excretory in function (because of their connection with storage and synthesis of glycogen and fat, deamination and urea formation these are considered analogous to the liver of vertebrates).

Morphology of Cockroach

Two species of cockroaches commonly found in India are Periplaneta americana and Blatta orientalis. Periplaneta americana is the largest and most common species.

Cockroaches are nocturnal and cursorial. Cosmopolitan in distribution, but more abundant in warm, humid areas. Cockroaches are brown or black-bodied. Bright yellow, red and green coloured cockroaches are found in tropical regions. Size of cockroaches ranges from 1/4 inches to 3 inches (0.6 cm to 7.6 cm). P.americana is 34-53 mm long. They are serious pests & vectors of several diseases.

Body is divided into the head, thorax and abdomen.

Head

Triangular, anterior-most region of the body, lies at right angle to the long axis. The distal narrow mouth bearing end is directed downwards (hypognathous). Head is covered by sclerites and bears sense organs, mouth parts and mouth.

All six sclerites are fused to form a capsule. Cervicum (neck, extension of prothorax) is present between head and thorax and allows movement in all directions with the help of muscles. Sense organs include two large kidney-shaped black compound eyes (formed of a number of ommatidia).

Antennae arise from membranous sockets which are made of many segments-podomeres. Antenna bears a thermoreceptor, moves in all directions, possesses bristles and sensory receptors that help in monitoring the environment. Mouthparts of cockroaches are mandibulate type or cutting & chewing type. Mouthparts consist of labrum (upper lip), labium (lower lip), maxillae (segmented), mandibles & hypopharynx. A median flexible lobe, acting as tongue (hypopharynx), lies within a cavity enclosed by mouth parts.

Thorax

Thorax consists of three segments-prothorax, mesothorax and metathorax. Thorax bears three pairs of jointed appendages and two pairs of wings on mesothorax and metathorax. Exoskeleton of each segment consists of four chitinous plates called sclerites which are joined by an articular/arthrodial membrane. The dorsal sclerite is called tergum or tergite. Ventral sclerite is sternum or sternite and two lateral sclerites are called pleura.

Each leg is formed by five segments, viz, coxa, trochanter, femur, tibia and tarsus (tarsus is made of five tarsomeres).

Cockroach has two pairs of wings. The first pair is called mesothoracic wings/tegmina (= elytra) and the second pair of wings are called metathoracic wings. In resting position metathoracic wings lie folded below the tegmina. Cockroaches do not fly, but wings help in escaping from danger.

Body

In adult cockroaches’ abdomen is made up of ten segments in both male and females. But in the embryonic stage eleven segments are present. In females, the 7th sternum is boat shaped and together with the 8th and 9th sterna forms a brood or genital pouch whose anterior part contains female gonopore, spermathecal pores and collateral glands. In males, genital pouch or chamber lies at the hind end of the abdomen bounded dorsally by 9th and 10th terga and ventrally by the 9th sternum. It contains dorsal anus, ventral male genital pore and gonapophysis.

Anal cerci, a pair of many jointed structures are present on the tergite of 10th segment in both sexes. These are sensitive to sound and other vibrations. Anal styles, a pair of small, spine-like unjointed structures are present on sternite of 9th segment in males only. They are sensitive to touch.

Morphology of Frog

It is called bullfrog due to its large size and loud sound. Its length is 17cm and breadth is 8 cm. The Indian bull frog is found in freshwater ponds, ditches, pools and under stones. The frog is poikilothermic and undergoes hibernation (winter sleep) and aestivation (summer sleep). Female lays eggs (spawn) and male sheds sperms on them. It can change colour to hide from enemies (camouflage). This protective colouration is called mimicry.

Skin is smooth and slippery due to the presence of mucus. Colour of dorsal side of the body is generally olive green with dark irregular spots. On the ventral side the skin is pale yellow. Skin is always maintained in moist condition. The integument is most useful in cutaneous respiration and absorbs water because the frogs do not drink water.” Shed skin once in a month during active life (moulting).

Body of the frog is divided into a head (flat and triangular) and trunk which is perfectly streamlined and most suited for aquatic life; the neck and tail are absent. Above the mouth a pair of nostrils are present. The eyes are bulging and guarded by upper eyelid (stationary), lower eyelid (movable) and transparent nictitating membrane. Nictitating membrane protects the eyes when a frog is in water. On either side of the eyes a membranous tympanum (ear) is present which receives sound signals.

Forelimbs are short without web and end in four digits; the hind limbs are long and muscular with web in between the toes and end in five digits. The hind-limbs are larger and muscular than forelimbs. The limbs help in swimming, walking, leaping and burrowing. The cloacal aperture is present in the midventral line between thighs. Sexual dimorphism is exhibited by the presence of vocal sacs and copulatory pads on first digits of forelimbs in males. Males are slender and darker than females.