ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 11. PHYLUM MOLLUSCA (Molluscs)

4. Class Gastropoda (Gastpods)

The Class Gastropoda comprises about 90,000 species, of which over 500 live in Ukraine. The largest gastropods in Ukraine are the rapa whelk (shell up to 12 cm high), the Roman snail (shell up to 8 cm high), and certain slugs whose body length reaches 10 cm.

Among marine gastropods, littoral forms predominate, although some inhabit considerable depths. In seas, particularly the Black Sea, the Mediterranean limpet (*Patella*) is widespread (shell diameter up to 25 cm). It firmly attaches itself to coastal rocks using its muscular FOOT.

The great pond snail (or staghorn snail), which reaches sizes of 68-70 mm, is found in ponds, small lakes, and old riverbeds. Using its muscular foot, it moves not only along solid substrates but also across the surface tension film of Water. It breathes atmospheric

air using a lung. When water temperatures drop to 6-7°C, the pond snail begins to respire oxygen dissolved in water, at which point its lung Functions as gills. In terms of feeding habits, the great pond snail is an omnivore, consuming PLANT AND ANIMAL matter as well as carrion.

Pond snails, marsh snails, and Bithynia are common in small freshwater bodies. For instance, ramshorn snails feed on microscopic Algae, making them welcome inhabitants of aquariums as biological cleaners. All these Molluscs can withstand the drying up of water bodies by secreting a dense film that seals the shell opening. Terrestrial molluscs tolerate significant Temperature fluctuations. In southern latitudes during summer, and in northern latitudes during winter, they enter a state of anabiosis. Anabiosis (from Greek *anabiosis* — return to life) is a state of an Organism in which life processes are suspended; an adaptation of the organism to unfavorable environmental conditions.

Gastropods are the only class of molluscs that have mastered not only aquatic environments but also land. A terrestrial lifestyle is led by slugs, typical representatives of which include the forest, field, and garden slugs. A characteristic feature of these molluscs is the absence of a shell, which is vestigial in them, preserved as a small oval calcareous plate overgrown externally by the mantle. Slugs breathe atmospheric air using a lung, which is a respiratory cavity beneath the mantle. The Skin contains numerous mucous glands, the secretion of which has a protective function. These animals perish under conditions of low humidity and high temperature, which is why they lead a crepuscular and nocturnal lifestyle, inhabiting biotopes with adequate moisture. Under favorable conditions, slugs reproduce intensively throughout the summer, laying 500 eggs in the soil and various shelters. From these, young hatch in 2-3 weeks and reach sexual maturity in just a half-month. They spend the winter in a state of torpor, burrowing into the soil to a depth of up to 0.5 m.

Terrestrial molluscs such as the forest slug and Roman snail are widespread in Ukraine. They feed on tree leaves and are thus classified as phytophages by feeding type. Eggs are laid in a small depression dug with the foot. Over the summer, there are 2-3 clutches of 20-25 eggs each. In autumn, with a drop in temperature to 8-10°C, they enter hibernation.

The body of gastropods is asymmetrical, with three well-defined body regions: the HEAD, visceral mass (trunk), and foot with a broad sole (Fig. 37). Gastropods move by means of a muscular foot, aided by mucus secreted on the sole. The head bears 1-2 pairs of tentacles. In the Roman snail, for example, the shorter first pair of tentacles (anterior tentacles) serves as an Organ of Smell, while the tips of the longer tentacles (posterior tentacles) bear the eyes. In most species, Sense Organs are well developed: eyes, statocysts, and osphradia.

In most gastropods, the visceral mass is enclosed in a spirally coiled shell (1.5-250 mm). The opening of the shell, into which the foot and head are retracted in times of danger, is called the aperture. In some species (such as the rapa whelk), the aperture is sealed by a special sturdy operculum made of horny material for protection against predators. In some gastropods, the shell is underdeveloped or entirely reduced (e.g., in slugs).

Fig. 37. Roman snail (after removal of the shell and mantle):

1 — anterior tentacles; 2 — posterior tentacles; 3 — gonad; 4 — "Liver";

5 — foot; 6 — Kidney; 7 — Heart; 8 — pulmonary vein; 9 — mantle edge.

The Oral Cavity of gastropods contains horny jaws and a muscular Tongue covered with rows of sturdy chitinous Teeth that form a radula (Fig. 38).

Fig. 38. Radula of molluscs.

Gastropods possess well-developed Salivary Glands. The intestine forms a loop. The anus is located above the head or lateral to it on the right side of the body.

The respiratory organs for most are gills (feather-like thin-walled outgrowths containing numerous Blood Vessels); in some, it is a lung (a specialized mantle cavity whose walls contain a rich network of blood vessels).

The Heart in most gastropods is two-chambered, consisting of an atrium and a ventricle.

Reproduction is sexual. Marine forms are predominantly dioecious, while freshwater and terrestrial (pulmonate) forms are hermaphroditic. Fertilization is internal. Pulmonates can reproduce via cross-fertilization, much like earthworms. Development is direct (in terrestrial and freshwater species) or involves metamorphosis (in marine species). In the latter case, a larva known as a veliger hatches from the egg and leads a planktonic lifestyle. Thus, the larvae facilitate the dispersal of the species.

Gastropods are an essential component of ecological communities, serving as a vital food base for aquatic and terrestrial animals.

Significance of gastropods in human life:

Positive significance:

1. Some species of gastropods (such as the edible snail and whelk) are consumed by humans.

2. Shells are used as souvenirs, and buttons are made from their nacreous layer.

Negative significance:

1. During years of mass reproduction, slugs cause significant damage to cereal crops, as well as to garden and orchard plants.

2. Many species of gastropods (such as the freshwater snail and the dwarf pond snail) serve as intermediate hosts for parasitic worms.

3. The venom of the salivary glands (for example, in the tropical mollusk of the genus Conus) causes fatal poisoning.