ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 11. PHYLUM MOLLUSCA, OR MOLLUSKS (Mollusca)

Interesting to know

- Among Mollusks, There is a species that ranks among the longest-lived of all known invertebrates. This is the freshwater pearl mussel, which can live up to 100 years in an aquarium. Other mollusks have a much shorter lifespan, ranging from several months to 20 years.

- In 90% of gastropod species, the shell is coiled dextrally (to the right), and only in 10% sinistrally (to the left). The reason for this phenomenon remains unknown. Shell growth is irregular and depends on the season and environmental conditions. As a result, growth rings are formed, which can be used to determine the age of the mollusk.

- Conchiolin is an organic substance (a protein) characteristic exclusively of mollusks. The Middle layer of the shell is formed by calcareous prisms oriented perpendicularly to the body surface. The inner layer consists of individual nacre, or mother-of-pearl, platelets (limestone crystals) arranged parallel to the surface and cemented together by conchiolin. It is conchiolin that imparts various shades to the nacreous layer, which can be snow-white, yellowish, greenish, or grayish. The shimmer and play of colors in mother-of-pearl are caused by The phenomenon of light Interference.

- Since ancient times, giant clam (Tridacna) shells have been used to make Water vessels, axes, and fishhooks.

- Purple dye murex snails possess a purple gland in their mantle. Since antiquity, the dye known as Tyrian purple was extracted from them. Obtaining just 1.5 g of purple required Processing 12,000 murex snails. Therefore, it is hardly surprising that fabrics dyed with purple were extraordinarily expensive.

- Most Representatives of the Class Bivalvia are capable of producing pearls. If any foreign particle (such as a grain of sand or a tiny Organism) accidentally lodges between the mantle and the shell, it becomes encased in alternating layers of nacre and conchiolin. This is how a pearl is formed, which is typically gray. Only in a few species do pearls possess a regular shape and attractive coloration (white, yellow, red, or black).

- In China, Japan, and several other countries, mollusks are artificially farmed by implanting various small objects under their shells to stimulate pearl development.

- The giant clam produces pearls weighing up to 7 kg, though they hold no jewelry value.

- In ancient times, byssus was a traded commodity: it was used to weave luxurious fabrics, which were eventually supplanted by silk imported to Europe from the East.

- When pursued by predatory fish, small squids (20-25 cm in length) inhabiting surface waters can leap into the air. Expelling a jet of water from their siphon with tremendous force, they leave their aquatic habitat and soar to a height of 5-10 m above the sea surface, occasionally landing on the decks of ocean liners. The flight distance in some cases reaches 50-60 m, and even further with a tailwind.

- The giant squid has an eye diameter of up to 40 cm.

- The octopus is capable of purposeful tool use: sometimes, holding a stone in one of its tentacles, it watches a pen shell (Pinna) until the bivalve opens its shell, after which the octopus wedges the stone between the Valves to prevent them from closing, and then devours its victim.

From the World of Science

- Research has shown that a mussel 5-6 cm in length can filter about 3.5 liters of water per hour.

- In the intertidal zone, mussel beds are exposed twice a day during low tide. At this time, they tightly seal their valves, allowing them to go without water for 5-6 hours.

The Evolutionary Process

- Chitons (Polyplacophora) share certain features with polychaete worms: a trochophore larva, the type of Nervous system (orthogon), the absence of eyes and tentacles, and metameric (repeating) Organs such as shell plates and gills.

- Primitive bivalves possessed a single, solid shell. However, the transition to living on soft substrates necessitated lateral body protection. In the course of evolution, the shell folded in half and split into two valves; consequently, the HEAD was reduced and oral palps developed.

- Primitive genera of gastropods retain bilateral body Symmetry, as seen in limpets.