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The basic tissues mentioned above organize to form organs which in turn associate to form organ systems in multicellular organisms. Such an organization is essential for more efficient and better-coordinated activities of millions of cells constituting an organism.
Each organ in our body is made of one or more types of tissue. For example, our heart consists of all four types of tissues, i.e., epithelial, connective, muscular, and neural.
We also notice, after some careful study that the complexity in organ and organ systems displays certain discernable trends.
This discernable trend is called the evolutionary trend (You will study the details in class XII). You are being introduced to the morphology and anatomy of three organisms at different evolutionary levels to show their organization and functioning.
Morphology refers to the study of form or externally visible features. In the case of plants or microbes, the term morphology precisely means only this.
In the case of animals this refers to the external appearance of the organs or parts of the body. The word anatomy conventionally is used for the study of the morphology of internal organs in animals.
You will learn the morphology and anatomy of earthworms, cockroaches, and frogs representing invertebrates and vertebrates.
7.3 EARTHWORM
The earthworm is a reddish brown terrestrial invertebrate that inhabits the upper layer of 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 fecal deposits known as worm castings. The common Indianearthworms are Pheretima and Lumbricus.
7.3.1Morphology
Earthworms have long cylindrical bodies. The body is divided into more than a hundred short segments which are similar (metameres about100-120 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). The anterior end consists of the mouth and the prostomium, a lobe that 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 –prepatellar, clitellar, and postclitellar segments (Figure 7.9).
Four pairs of spermathecal apertures are situated on the center-lateral sides of the intersegmental grooves, i.e., 5th -9th segments.
A single female genital pore is present in the mid-ventral line of the 14th segment. A pair of male genital pores are present on the ventrolateral sides of the 18thsegment.
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.
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