Mollusks and Arthropods: Characteristics and Classification
Mollusks: Characteristics and Classification
Mollusks are invertebrates of great importance to humans, providing food and materials for jewelry, among other things. A negative aspect is that some can cause schistosomiasis.
Mollusk Classification
- Monoplacophora: Single shell, found in deep-sea environments.
- Polyplacophora: Shell composed of eight plates, marine.
- Scaphopoda: Conical shell, buried in the seabed sand.
- Gastropoda: May have no shell or a single shell; the most numerous and diverse group (snails, sea slugs, etc.).
- Pelecypoda (Bivalvia): Two shells (e.g., oysters).
- Cephalopoda: Most evolved, with organs resembling those of vertebrates (octopus, squid).
Mollusks are found in terrestrial, freshwater, and marine environments.
Mollusks have soft bodies, bilateral symmetry, and lack segmentation. They are triploblastic, protostomes, and coelomate (with heterolecithal eggs). Their bodies are divided into three regions: head, foot, and visceral mass. The visceral mass is almost always covered by a mantle, which often secretes a shell.
The Mollusk Shell
Some mollusks have shells, while others (like squid and octopuses) do not. In squid, the shell is internal, sometimes referred to as a “pen.” The shell can be single or composed of multiple parts, and it may be coiled (as in snails).
The shell is formed by the secretion of the mantle. It is divided into three layers:
- Internal: Calcareous, smooth, often with mother-of-pearl.
- Middle: Crystalline.
- External: Thin and organic.
Mollusk Physiology
- Nutrition: Extracellular digestion occurs in the complete digestive tract (U-shaped or coiled), followed by intracellular absorption. Digestion is mixed. The mouth contains a radula, a structure with tiny teeth used to scrape food. Some mollusks obtain nutrients through filter-feeding.
- Nervous System: Ganglionated, with three main ganglia (cerebral, pedal, and visceral) that send nerves to various parts of the body. These ganglia have sensory, motor, and vegetative functions. Mollusks also possess sensitive structures, such as eyes.
- Excretion: Performed by primitive kidneys, which remove metabolic waste from the pericardial cavity.
- Respiration: Gas exchange occurs through the epidermis, mantle, or a form of lung (in the mantle cavity). Most mollusks breathe through gills.
- Circulation: Open circulatory system in most mollusks; cephalopods have a closed circulatory system. Blood is pumped by a heart, and a cavity distributes nutrients and oxygen.
- Reproduction: Usually dioecious (separate sexes) or hermaphroditic, most are oviparous with indirect development. Reproduction is exclusively sexual.
Gastropods have an ovotestis (acting as both ovary and testis, but they do not self-fertilize). Pelecypods (e.g., oysters) and Scaphopods lack a distinct head. The head typically has tentacles and eyes. The foot is muscular and adapted for various functions, such as digging.
Arthropods
Arthropods are the most numerous animal phylum. They are characterized by: articulated appendages, bilateral symmetry, segmented bodies, and an exoskeleton. Arthropods play various roles, including in food (shrimp, lobster), pollination, and disease transmission. They range in size from microscopic to the giant Japanese spider crab. Insects are generally not very large, partly because they fly and their exoskeleton is heavy.
Arthropod Classification
- Insects: Head, thorax, abdomen; 3 pairs of legs; 2 antennae; wings; two mandibles. (e.g., bee, fly, cockroach)
- Crustaceans: Cephalothorax, abdomen; 5 or more pairs of legs; 4 antennae; one pair of mandibles. (e.g., crabs, lobsters)
- Arachnids: Cephalothorax, abdomen; 4 pairs of legs; chelicerae and pedipalps. (e.g., spiders, scorpions)
- Chilopoda (Centipedes): Head, trunk; 1 pair of legs per segment; 2 antennae; 2 mandibles.
- Diplopoda (Millipedes): Head, trunk; 1 pair of antennae; 1 pair of mandibles; 2 pairs of legs per segment.