**1. Introduction**

Extensive research on invertebrate immune system for the last few decades, including mol‐ luscs, revealed that invertebrates contain peptides which are endowed with anti‐microbial activity [1]. These peptides can trigger specific anti‐bacterial reaction by producing differ‐ ent isoforms specific for each bacterial species. Among immunological molecules of inverte‐ brates, Toll‐like receptor 4 (TLR4) gained much attention, though its essentiality happens to be more pronounced in vertebrates [1]. Gastropod diversity is well documented, recording 40,000–150,000 species with size variance of 1 mm to 1 m and indicating a strong immune system in gastropods [1–3].

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The giant African snail, *Achatina fulica*, is one of the large and most widely distributed land snails, considered as an agrihorticultural pest [4].Since *Achatina* develops rapidly and produces large numbers of offspring, it is now listed as one of the top 100 invasive species in the world [5]. Moreover, *Achatina* is a unique species and maintains three different life cycle stages in the same individual, surviving in the environment for millions of years. Apart from maintaining the critical life cycle stages, *A. fulica* has survived successfully, consequently, gaining the disre‐ pute as an agricultural pest in India. In addition, these snails are considered as bio‐indicators of ecosystem health. Although they do not possess immunoglobulins, they have evolved unique modalities to detect and respond to microbial surface antigens such as lipopolysaccharides (LPS), lipoteichoic acids, lipoproteins, peptidoglycans and (1→3) β‐d‐glucans [6].

Terrestrial snails are well known for accumulating heavy metals in their tissues and serve as a pertinent species for monitoring trace metals, agrochemicals, urban pollution and elec‐ tromagnetic exposures [7]. The effect of accumulated heavy metals in different molluscan tissues and possible use of such alterations as biomarkers of exposure to xenobiotics has been investigated in some detail [8, 9]. Although snails are considered as alleged pest they are used by humans for various purposes including vigorous consumption of mollusc meat in several countries around the globe, including tribal and urban populations of India and Bangladesh [10]. Another important aspect is the ethno‐medicinal use of several mollusc species high‐ lighted by several authors [11, 12]. Pharmacological application of different body parts of mollusc are used to treat several diseases which suggests its potential to act as a source of drug [12]. In the present chapter, various characters of *Achatina* will be described including their unique immune system that contributes toward the evolutionary success of *A. fulica* in the terrestrial ecosystem.
