Mollusks

MOLLUSKS, or soft-bodied animals (Mollusca), are a well-defined phylum of invertebrate animals. The body is soft, unsegmented, and typically bears a shell. The integument forms a mantle fold that covers the body or fuses at the edges with its surface. The mantle secretes a hardening substance that forms the shell. In cross-section, the latter consists of an inner nacreous layer, a middle porcelain layer, and an outer leathery layer. The mollusk's body is partially attached to the inner surface of the shell. In pearl oysters, the nacreous substance can layer over sand grains or parasites located between the shell and the mantle; thus, pearls are formed. Mollusks move by means of a flat, sole-like (Fig. 2) or wedge-shaped (Fig. 5) "foot." The muscles are smooth; movement is very slow. The most important classes of mollusks are: a) bivalves, or lamellibranchs (Lamellibranchiata), with a bivalve, usually symmetrical shell (Fig. 1); b) gastropods, or snails (Gastropoda), with the ventral side of the body transformed into a "foot"; the shell is spirally coiled, asymmetrical (Fig. 2, 4, 7-15), and may be reduced; and c) cephalopods (Cephalopoda, for example, the squid Loligo), the most highly organized mollusks with a well-defined head bearing 8-10 appendages covered with suckers, called tentacles; they swim rapidly (Fig. 3). Mollusks feed on either plant food or plankton, or are predators. They typically live in water, marine or fresh; many species are terrestrial. They are dioecious or hermaphroditic. In hermaphrodites, cross-fertilization is usually observed. In many species of mollusks, larvae emerge from the eggs.

Figure 1. Swan mussel (Anodonta). Figure 2. Pond snail—Limnaeus stagnalis: 1—eyes; 2—foot; 3—respiratory pore; 4—oral lobes; 5—tentacle. Fig. 3. Cuttlefish—Sepia officinalis. Figure 4. Garden snail—Helix pomatia. Figure 5. Mussel—Mytilus edulis: 1—foot; 2—anal siphon; 3—mantle edge; 4—threads of the byssus attachment substance.

For mollusk larvae, the early formation of the shell primordium is characteristic. Typical mollusk larvae resemble the trochophore of annelid worms. Some mollusks are viviparous. Importance of mollusks. Benefits: a) some mollusks are edible—oysters (Ostrea edulis), cockles (Cardium edule), mussels (Mytilus edulis, Fig. 5), all from the bivalve mollusks; the Roman snail (Helix pomatia, Fig. 4); some cephalopods (octopuses) are also used for food (the frutti di mare of the Italians); some gastropod and cephalopod mollusks can be feed for pigs; their role as food for fish is important; b) mollusk shells can be used for obtaining lime salts and various crafts (buttons, etc.); c) from some marine and river mollusks, pearls are obtained (Margaritina margaritifera); d) from the "ink sac" of the cuttlefish, the pigment sepia is obtained (Sepia officinalis; Fig. 3); e) fossilized mollusks play the role of index organisms for determining the geological age of strata and deposits. Harm of mollusks. 1. Toxicity of mollusks. Some species are primarily poisonous because they possess poison glands; such are, for example, Dolium or Tritonium (Mediterranean Sea), whose salivary glands secrete 4% sulfuric acid, which serves to dissolve the calcareous armor of echinoderms that serve as food for the mollusks. In the case of the octopus, the parasite passes from the first host (mollusk) to the second (crab, fish) when the first is eaten by the second. The role of mollusks in the epidemiology of human trematodoses comes down to a) the infestation of water or aquatic plants by larvae emerging from...

Figure 6. Oncomelania nosophora. Figure 7. Bythinia striatula. Figure 8. Melania libertina. Figure 9. Limnaea truncatula. Figure 10. Bullinus contortus. Figure 11. Bullinus Dybowskii. Figure 12. Physopsis africana. Figure 13. Planorbis metidjensis. Figure 14. Segmentina hemisphaerula. In the octopus (Octopus, Cephalopoda), the posterior pair of salivary glands is poisonous; their secretion paralyzes crabs. The described forms have no practical significance for humans. When living in polluted water, some mollusks, for example, mussels (Fig. 5), Cardium, and oysters (Fig. 15), acquire harmful properties and, when consumed as food, cause poisoning (see Mytilotoxin). Prevention of mollusk poisoning consists of keeping the mollusks in clean seawater for a long time. 2. Mollusks as transmitters of infections. A number of authors have shown that oysters, when kept in seawater with typhoid bacteria, became ill and died. Eating infected oysters can lead to a person contracting typhoid fever. Bacteria can enter oysters from the ice in which they are stored. 3. Mollusks as intermediate hosts of helminths. This role of mollusks is great and significant, particularly in relation to the epidemiology and prevention of helminthic diseases in humans and animals. Some stages of the life cycle of various flukes (Trematoda) take place in mollusks. Human flukes have only gastropods as their intermediate hosts among mollusks. Trematode larvae actively penetrate into mollusks and also actively leave them in the corresponding phase of development. In the case where there are two intermediate hosts, the parasite passes from the first host (mollusk) to the second (crab, fish) when the first is eaten by the second. The role of mollusks in the epidemiology of human trematodoses comes down to a) the infestation of mollusks by parasites; b) the infection of second intermediate hosts that serve as food for humans. In the above relations, species of freshwater mollusks are of special medical-parasitological interest: class Gastropoda, order Prosobranchia, family Rissoidae. Oncomelania (Katayama) nosophora (Fig. 6) (Japan)—intermediate host of the dioecious fluke (Schistosomum japonicum). A similar role in China is played by O. hupensis, family Bithyniidae. Bythinia striatula (Fig. 7) (Japan, China to the Amur)—first intermediate host of the Chinese liver fluke (Clonorchis sinensis), the second being fish of the family Cyprinidae, which serve as food for the mollusks. Family Melaniidae. Melania libertina (Fig. 8) (Japan, China, Korea, Formosa)—first intermediate host of the lung fluke (Paragonimus Ringeri); Melania extensa (Korea) is also an intermediate host of the trematode Metagonimus Yokogawai. Family Ampullariidae. Ampullaria luteostoma (Venezuela)—first intermediate host of Paragonimus. Order Pulmonata (lung-breathing). Family Limnaeidae. Limnaea stagnalis—great pond snail (Fig. 2) (USSR) and L. truncatula (Fig. 9)—dwarf pond snail (USSR), intermediate hosts of the liver fluke (Fasciola hepatica). L. natalensis is also for F. gigantica and Schistosomum haematobium. Family Bullinidae, Bullinus contortus (Fig. 10) (Southern France, Corsica, Sicily, North Africa, Egypt) and B. Dybowskii (Fig. 11) (Tunisia, Egypt)—intermediate hosts of Schistosomum haematobium. Physopsis africana (Fig. 12) (Africa)—intermediate host of Sch. haematobium and Sch. Mansoni. Family Planorbidae, Planorbis metidjensis (Fig. 13) (Portugal, Spain, Morocco, Algeria)—intermediate host of Sch. haematob. and Pl. boissyi (Egypt)—the same for Sch. Mansoni. Segmentina Schmackeri (Fig. 14) (East China), S. hemisphaerula (China), and S. nitidella (Japan)—intermediate hosts of Fasciolopsis buski.

Figure 15. Oyster with one shell valve removed: 1—anterior end of the mantle; 2—cross-section of the adductor muscle; 3—gill lamellae. 4. Technical harm of mollusks. Teredo navalis (Lamellibranchiata)—the "shipworm," bores into wood (piles, ship hulls, dams), making it a serious pest; the "rock borer," Pholas, which drills even into stones, possesses the same property. The zebra mussel (Dreissena polymorpha), which has adapted to freshwater life, sometimes colonizes water pipes to such an extent that it completely clogs them and stops the water supply. The fight against mollusks is difficult. Cleaning water bodies is not a guaranteed measure, as complete cleaning is technically impossible in many cases. Draining water bodies is applicable only to a limited number of them, and furthermore, mollusks equipped with an operculum can remain without water for a long time. Copper sulfate in a solution of 1:500,000 to 1:2,000,000 kills the mollusks that are intermediate hosts of the liver fluke; but such a concentration of the solution is harmful to some fish, especially their fry. Lime in a concentration of 0.1:100 is lethal to the mollusks that are intermediate hosts of bilharzia (Schistosomum).