INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION SPIRALIA

PHYLUM FLATWORMS (PLATHELMINTHES)

SUBPHYLUM TURBELLARIA, OR ARCHIDERMATA

SUPERCLASS ARCHOPHORA

CLASS ACOELOMORPHA

Small marine organisms, a few millimeters in length (Fig. 115). They predominantly inhabit the littoral zone (the coastal strip of the seafloor) and sublittoral zone. Some species belong to the interstitial fauna (living in the narrow spaces between sand grains) and possess an elongated body.

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Fig. 115. The acoel turbellarian Convoluta (from Dogel): a - longitudinal section; b - general view:

1 - parenchyma; 2 - Mouth; 3 - digestive parenchyma; 4 - genital openings; 5 - male copulatory organ; 6 - eyes; 7 — statocyst

The integumentary-muscular sac in acoels is indistinctly separated from the parenchyma; they lack a gut, retaining only a mouth and a Pharynx leading into the parenchyma, where specialized Cells—phagocytes—are clustered to handle Digestion. Protonephridia are absent. They also lack a well-formed Reproductive System: Germ Cells are either diffusely scattered throughout the parenchyma or gathered into small groups; there are no efferent ducts, only a copulatory organ.

Many species consume diatoms, while others are predatory, such as Convoluta convoluta, which preys on small animals that adhere to a web formed by mucous threads secreted by these worms. Some species form symbiotic relationships with zooxanthellae—flagellates that live in the Connective Tissue of convolutids, finding shelter there, utilizing carbon dioxide and nitrogen compounds, and supplying the worms with photosynthetic products (Polysaccharides) and oxygen.