MOLLUSCA

After insects, mollusks are probably the most family invertebrates. In numbers and diversity, they are second only to the arthopods with probably at least 100,000 to 120,000 species are grouped into the Phylum Mollusca. Mollusks are unsegmented coelomate animals that have an anterior heads, a ventral foot, and a dorsal visceral mass covered by a mantle. The mantle secretes a calcareous shell and encloses a mantle cavity containing paired ctenidia or gills.

Of all the invertebrates, mollusks probably have been and still are most valued by man. They are a major food source in many parts of the world, their shells provide a variety of products, and the diverse and beautiful forms that these may take have led to mollusks becoming important elements in the art, culture and traditions of many races.

In 300 B.C., Aristotle divided animals into two major groups, one is Enaima (with red blood), the other is Anaima (without red blood). The animals with shell called Testacea, the other group without shell called Cephalopoda (Smith et al. 1971). Linnaeus categoried the anaima animals with soft body into two groups, such as Mollusca (with inner shell or without shell) and Testacea (with outer shell) in the tenth edition of his Systema Naturae (Linnaeus 1758). There are three groups within Testacea i.e., Multivalvia (multi-shells), Bivalvia (two-shells) and Univalvia (one-shell). Lamarck renamed the Enaima and the Anaima as the Vertebrate and the Invertebrate animals.

CLASSIFICATION OF MOLLUSCA

Mollusks are a very diverse group of animals such as abalones, limpets, turbans, conchs, cowries, cones, snails, slugs, tusks, chitons, clams, mussels, scallops, oysters, venuses, squids, octopuses, cuttlefishes and nautilus. Although some groups are so modified that they no longer have a dorsal shell or a mantle cavity, all mollusks retain the basic head-foot-visceral mass morphology diagnostic of the Phylum Mollusca. The head carries out sensory and feeding functions. The foot is basically a muscular structure which functions as a locomotory organs. The visceral mass comprises the major internal organ systems: heart, gonads, kidneys, digestive system, etc. No unique characters define the entire Phylum Mollusca, which shows great structural and ecological variability, and members of the group can only be separated from other animals by the possession of a combination of characters (Beesley et al. 1998). Eight classes of living mollusks have been recognised (Fig. 1)﹕

Class Caudofoveata

Also termed the Chaetodermomorpha, are worm-shaped, cylindrical, and covered by a cuticle and aragonitic scales. The posterior mantle cavity contains one pair of ctenidia. The ventral gliding area is reduced and lacks a distinct pedal groove. Chaetoderms burrow in marine muddy sediments, and feed on microorganisms.

Class Solenogastre

Or Neomeniomorpha, are worm-shaped and laterally compressed. The foot lies in a distinct ventral groove, the mantle has a cuticle bearing aragonitic scales or spicules, and the modified mantle cavity lacks ctenidia. These marine carnivores are epibenthic or epizoic on cnidarians, on which they feed. Proneomenia was collected from the South China Sea.

Class Polyplacophora

Chiton or coat of mail shells have a flattened body and a broad foot, a mantle covered with cuticle and spicules or scales and a shell of eight valves. The shell valves bear sensory papillae or aesthetes. Most chitons are grazing herbivores, some feed on colonial organisms, such as sponges, and one group captures small prey.

Class Monoplacophora

Monoplacophora are covered with a cap-shaped shell. The peripedal mantle cavity supports five to six pairs of modified ctenidia. There are five to six pairs of excretory organs. The animals are marine detritus feeders, attached to hard substrata. The living fossil, Neopilina galatheae (Lemche, 1957) was discovered in 1952 (Lemche and Wingstrand 1959). Class Scaphopoda

The tusk shells are conchiferans in which the mantle is fused mid-ventrally and the tubular shell is open at both ends. Thead bears a long snout and two groups of slender tentacles, termed captacula, and the foot is cylindrical and pointed. These marine micro-carnivores burrow in sediment.

Class Bivalvia

Bivalves are laterally compressed conchiferans covered with a shell composed of two valves jointed dorsally by a ligament. The mantle cavity surrounding the body contains a single pair of enlarged ctenidia, and the posterior mantle is often extended into siphons. They are benthic, often burrowing, or may be epiphytic and they occupy a wide variety of marine and freshwater habitats.

Class Gastropoda

In all gastropods, including snails, slugs and limpets, the mantle cavity is rotated from its posterior position during the developmental process of torsion, so that it comes to be at the anterior end. Gastropods use a wide variety of feeding methods and occur in many types of terrestrial, freshwater and marine habitats, including the pelagic zone. This class is by far the largest group of Mollusca.

Class Cephalopoda

They are conchiferans with a dorso-ventrally elongated body. When present, the shell is coiled to straight, typically chambered such as Nautilus. The head is free, and bears a pair of eyes, and one or two circles of eight to ten (or up to about 90 in Nautilus) tentacles surrounding the mouth. The foot is modified as a funnel for jet propulsion. A rectal ink sac provides fluid used in defence. Among these marine, benthic or pelagic carnivores are the largest mollusks, with body sizes up to 20m.

*SHELL OBSERVATION

Gastropods observation

There are two methods to observe the shell, firstly, hold shell with care, apex in up position and aperture face to the observer, now, you see the aperture, two posibilities：1) Dextral Shell－the aperture is in the right hand side of columella; 2) Sinistral Shell－the aperture is in the left hand side of columella (Fig. 2); secondly, followed the same method, but you see the apex. There are two posibilities as well：1) Dextral Shell－clockwise spiral from center to edge; 2) Sinistral Shell－counterclockwise spiral from center to edge.

Bivalves observation

Umbo up position, the posterior end face to the observer, now the shell in your right hand side is the right shell, the shell in your left hand side is the left shell (Fig. 3). You can also watch the inner surface of shell with umbo in up position, right shell－the pallial sinus is in your right hand side; left shell－the pallial sinus is in your left hand side.

*HOW TO MEASURE SHELL CHARACTERS

Bivalves measurement

Shell length (SL) is the horizontal distance from the anterior end to the posterior end of the shell. Shell height (SH) is the vertical distance from the umbo to the ventral margins of the shell. Shell breadth (SB) is the widest distance when the two valves are closed (Fig. 4).

Gastropods measurement

Shell length (SL) is the vertical distance from the apex to the anterior canal end of the shell. Shell width (SW) is the widest distance of the shell when the aperture view face to you (Fig. 5).

THE NAME OF SHELL

The names of each part of gastropods shell is shown in Fig. 6 and of bivalves is shown in Fig. 7.