Invertebrates Facts

  • Invertebrates are multi-cellular and all the cells have different responsibilities to keep the animal alive.
  • Invertebrates have no backbone or bony skeleton or cells wall like all other animals.
  • Invertebrates like echinoderms do not have heads.
  • There are millions of invertebrates living in our house in the name of dust mites which are hardly visible to human eyes.
  • Invertebrates under a process called metamorphosis through which they change form as they grow.


The phylum Acoelomorpha is a species with planula - like features and is also considered to belong the Platyhelminthes phylum for a long time. But it is recently considered to be a separate phylum. This was coined by Jaume Baguna and Marta Riutort.

Acoels are fully considered to be a marine living organism. It is living in the grains of sediment, it can Swim like a plankton or it can crawl on an algae. Their bodies are too soft that it cannot be classified easily. Acoels have a statocyst which helps them in orientation to the gravity.

Acoels are a flattened worm; it is too small less than 2 mm in length. They do not have guts in their body. Digestion can take place by syncytium. Syncytium forms a vacuole around the food ingested into the worm.

Other bilateral animals have guts and the lining of epithelial cells, but these two things are absent in Acoels. To compensate this small pharynx can leaning from the mouth to the vacuole. Acoels seen to be a flat bodied organism.

Acoelomorphs considered resembling flatworms for the following reasons. Like flatworms Acoels don't have guts, circulatory system, respiratory system and excretory system. They are also not having true brain or ganglia.

Alternatively it has a network of nerves beneath the epidermis. Statocyst act as a sensory organ and the light can be detected by the Acoels with the help of primitive pigment called ocelli. These species are hermaphrodites without gonads and there is no duct in the female reproductive system.

Instead of gonads, the gametes are produced from the mesenchymal cells which can fill the body from epithelial cells to digestive vacuoles.

Features of Acoelomorpha

  • Acoels are hermaphrodites that is male and female reproductive organs are present in the same organism and they are mutually inseminate in sexual reproduction
  • Acoels have a solid body structure lacking of coelom, pseudocoel or hemocoel between body wall that is seen in other bilateral animals
  • Acoels having soft body without cuticle or other skeletal support
  • They are not having stem cells which give rise to various other types of cells in the body
  • They are having a pair of flagella arising from the centrioles

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