Invertebrate Sensory Systems and Skeletal Structures

Invertebrate Sensory Organs

Vision

Photoreceptors are grouped. In cnidarians, they are simple eyespots. Flatworms possess more complex structures, resembling eyeglasses. Annelids and gastropods have more complex organs, sometimes featuring a lens (ocelli).

In gastropods, there are two types of eyes:

• Ocelli: These are sensitive only to light.
• Compound eyes: Structured with ommatidia, these simple units result in mosaic vision.

Crustaceans have two compound eyes mounted on mobile stalks. Cephalopods possess camera-type eyes, forming an eyeball containing two types of photoreceptors: cones and rods. They perceive light, shadows, and colors.

Organs of Hearing

Organs receiving sound waves are grouped. Tympanal organs are often derived from tracheal openings of the respiratory system. They consist of a membrane that vibrates upon receiving sound waves, transmitting this information to the brain.

Organs of Balance

These organs report on body position and its variations. Statocysts are composed of a hollow cavity lined with receptor cells. Inside the cavity is a spherical structure, the statolith, which moves freely. This mechanism allows the animal to orientate and navigate.

Skeletal System

The skeletal system varies significantly among invertebrates:

• Porifera: Small spicules made of spongin, calcium carbonate, or silica provide consistency and shape.
• Cnidaria: Covered with a calcareous external structure.
• Gastropod Molluscs: Possess an outer shell made of calcium carbonate.
• Bivalve Molluscs: Have two articulated shells or valves.
• Cephalopod Molluscs: Some possess an internal calcareous shell called a pen or cuttlebone.
• Echinoderms: Have an internal skeleton formed by calcareous plates.
• Arthropods: Have an external skeleton jointed and formed by chitin.
• Vertebrates: Have an internal skeleton formed by hard structures, which are bones.

Organs of Taste and Smell

These senses are often interrelated. Taste receptors are located near the buccal structures. Olfactory receptors are localized and related to reproduction, food seeking, and interaction.

Bodies of Touch

Receptors for touch are grouped and include several types: thermal, mechanical, and chemical. Scattered neurons function as nociceptors. Mollusks use specialized cells in their tentacles. Arthropods use antennae to sense water (H₂O).

Other Organs of the Senses

Some sensory organs are unique to specific animals:

• Lateral Line: Found in fish and amphibians. These are lines formed by small pores crossing the scales, connected to a fluid-filled tube running along the flanks, just beneath the skin. Inside, hair cells detect water vibrations.
• Facial Pit: A thermosensitive organ in snakes, formed by two structures between the eyes and mouth, which capture infrared radiation.

The Ampullae of Lorenzini are organs capable of capturing variations in electric fields, perceiving small electrical currents, particularly in cartilaginous fish (condrichthyes). Furthermore, some fish that undertake long migrations are guided by the Earth's magnetic field. Others detect echoes, allowing them to focus and move; this ability is called echolocation.