Planarian

Planarians are free-living flatworms that constitute the order Tricladida. They are characterized by a bilaterally symmetrical, dorsoventrally flattened body with a three-branched intestine.

These animals have remarkable regenerative abilities, and some species can rebuild an entire body from tiny fragments. This capacity arises from pluripotent stem cells called neoblasts, which can generate every major cell type in the body. As a result, planarians have become a key model organism for studying regeneration, stem-cell regulation, and body patterning.

They are mostly carnivorous scavengers that feed on invertebrates (like insect larvae, worms, and crustaceans) as well as dead or dying animals. They extend a muscular pharynx, often releasing digestive enzymes to soften prey before sucking it into their gut.

Description

These animals have soft, wedge-shaped bodies that may be black, brown, blue, gray, or white, depending on the species. They are typically 0.1 to 0.6 in (3 to 15 mm) long, though some species, like Bipalium kewense, may grow as long as 1 ft (30 cm).^[1]

Being flatworms, their bodies are bilaterally symmetrical, lacking a fluid-filled body cavity or coelom (thus acoelomates). Instead, the space between their organs is filled with soft parenchymatous or mesenchymal tissue.

A population of pluripotent neuroblasts is found in the parenchyma (accounting for 20 to 30% of all cells in an adult planarian). These cells are small (diameter: 5-10 μm) and possess a large nucleus. They divide mitotically and help regenerate missing tissues.

Organ System

• Planarians lack a circulatory and respiratory system. Thus, gaseous exchange occurs via diffusion through the body wall.
• Their mouth is on the ventral surface, near the middle of the body, which leads to a muscular pharynx. The pharynx leads to a three-branched intestine that runs across almost the entire body. Since they lack an anus, they expel waste through the same opening that serves as the mouth.
• They have a bilobed cerebral ganglion (brain) at the front of the body. From it, two ventral nerve cords run to the tail and are connected by transverse nerves, forming a ladder-like system. The head bears two light-sensitive ocelli (eyespots) and two auricles that detect touch and chemicals.

Taxonomy

All planarian species are classified into 3 suborders, 5 superfamilies, and 12 families.

• Suborder: Maricola
  • Superfamily: Cercyroidea
    • Family: Centrovarioplanidae
    • Family: Cercyridae
    • Family: Meixnerididae
  • Superfamily: Bdellouroidea
    • Family: Uteriporidae
    • Family: Bdellouridae
  • Superfamily: Procerodoidea
    • Family: Procerodidae
• Suborder: Cavernicola
  • Family: Dimarcusidae
• Suborder: Continenticola
  • Superfamily: Planarioidea
    • Family: Planariidae
    • Family: Dendrocoelidae
    • Family: Kenkiidae
  • Superfamily: Geoplanoidea
    • Family: Dugesiidae
    • Family: Geoplanidae

Regeneration

Many species, especially freshwater ones, such as Schmidtea mediterranea and Dugesia japonica, regenerate their body parts after injury. The process begins with wound-induced signals that activate local cellular responses and initiate tissue rebuilding. In fact, it involves both the formation of new tissue and the repatterning of existing tissues (morphallaxis).

The process occurs as follows:

• Neoblast cells near the wound become highly active and start dividing mitotically. They migrate toward the injury site to replace the missing tissue.
• The accumulating cells form a cluster known as the blastema.
• Muscle cells across the body act as a source of positional information (via pathways like Wnt and BMP) and instruct regenerating cells about which tissues should form and where.
• Using this positional information, blastema cells differentiate into the required structures, while existing tissues are simultaneously reorganized to maintain body symmetry and functional anatomy.

Studies show that a body fragment as small as 1/279^th of the planarian Girardia tigrina has the capacity to regenerate into a complete organism in a few weeks.^[2]

Reproduction

Depending on the species, planarians reproduce both sexually and asexually. A few species, such as Dugesia japonica, can reproduce both sexually and asexually.^[3]

Sexual

They are hermaphrodites, meaning each individual has both male and female reproductive organs. Typically, two planarians mate and exchange sperm (cross-fertilization).

After fertilization, the eggs develop inside the body and are enclosed in a tough protective case called an egg capsule. The capsule is then released and attaches to surfaces such as plants or stones. A few weeks later, the young hatch and gradually grow into adults.

Asexual

In this mode of reproduction, the planarian usually constricts its body and splits into two or more fragments, with each fragment then regenerating the missing parts.

Lifespan

Many freshwater planarians are often considered ‘biologically immortal’ because they can regenerate an entire body from small fragments owing to their neoblast cells. However, they are mortal in natural, suboptimal conditions, particularly when stressed by low food availability.^[4]

Thus, planarians can live indefinitely only in controlled laboratory conditions.