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PLATYHELMINTHES

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(Gegenbaur, 1859 )

Sections
  • Background
  • General Description
  • Development
  • Ecology
  • Zoogeography
  • Background

    Because of their compressed shape, platyhelminths are commonly known as ‘flatworms’. There are several suggested classification schemes for the phylum, but here it is divided into Classes Trematoda (Rudolphi 1808), Cestoda and Turbellaria Ehrenberg 1831. The first two classes are parasitic, while Turbellaria are mainly free-living, only a few being parasitic or commensal. Marine species in all of the classes have planktonic stages, for dispersal, and also for transfer between hosts in the case of parasitic species. However, in plankton samples, most stages of the parasitic species will only be observed inside various organisms. For example larval trematodes can sometimes be seen inside medusae and chaetognaths. Only turbellarians are likely to be sampled with any regularity, so are the only class described here.
     
    Marine turbellarians are usually found on or close to the sea bed, but can be carried higher in the water column under turbulent conditions. Juvenile and small specimens can occasionally be quite numerous in plankton samples. Most tubellarians are predatory on invertebrates smaller than themselves, while others are mainly herbivores, ectoparasites, or scavengers. Many are quite colorful when alive, but their bodies are often opaque and cream coloured when preserved and the internal organs are not usually obvious. They are generally flattened and leaf shaped with rounded corners (Figure 1A-C). Externally they usually have two clusters of black, light-sensitive organs (ocelli) on their upper surface.

    Further information: Ball & Reynoldson 1981; Hyman 1951; Larink & Westheide 2006; Prudhoe 1982; Smith et al. 2002.

     
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    General Description

    Many species are quite colorful when alive, but their bodies are often opaque and cream coloured when preserved and the internal organs are not usually obvious. Turbellarians are generally flattened and leaf shaped with rounded corners (Figure 1A-C). Externally they usually have two clusters of black, light-sensitive organs (ocelli) on their upper surface. They have no body cavity other than the gut (which may be lacking in the smallest species) and lack an anus. Food and waste pass through the same pharyngeal opening, which is generally located on the dorsal mid-body (Figure 1B). In some species the pharynx can be extended as a dangling tube with the mouth on the end. In larger flatworms the gut is often very highly branched, so that food energy can be transported to all parts of the body. The lack of a body cavity constrains flatworms to be flat, as they must respire by diffusion and no cell can be too far from the outside. They generally have aciliated epidermis and movement is controlled by longitudinal, circular, and oblique layers of muscle. Locomotion may be by coordinated waves of cilia on a secreted mucus trail, but some species can swim by rhythmic muscle contractions in an undulating pattern. The juvenile turbellarians usually encountered in plankton samples are often curled up and distorted and could only be classified by a specialist.
     
     

    Figure 1. Turbellaria. A) Typical juvenile body shapes from plankton samples; B) generalised adult structure; C) general adult appearance; D) structure of Müller’s larva; E Müller’s larva. (B-C unknown source; D after Hyman 1951; E from Trégouboff & Rose 1957).

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    Development

    Turbellarian development is usually direct (eggs hatch into tiny individuals that resemble the adults), but species in Order Polyclada have indirect development via a ciliated larval form that resembles an annelid trochophore. The commonest type is known as a Müller’s larva (Figure 1D, E), characterised by a variable number of posteriorly projecting lobes. Prior to metamorphosis there may be 7-10 lobes arranged over the body. Most have pigmented ocelli, 3 in early larvae, sometimes more in later larvae. The other rarer larval type is known as a Götte larvae, which only develops 4 lobes. Both larvae are completely ciliated, the cilia longer in particular regions, usually a longer tuft on the anterior and posterior ends. When alive they may be pigmented, mainly brown to orange. Metamorphosis transforms the swimming, particulate feeding larva, into a creeping, generally predatory adult. The body flattens, the larval lobes are gradually absorbed and the ocelli increase in number. Size: Müller’s larvae ~0.15-1.8 mm, post-larvae ~0.5-5.0 cm.

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    Ecology

    Marine turbellarians are usually found on or close to the sea bed, but can be carried higher in the water column under turbulent conditions. Juvenile and small specimens can occasionally be quite numerous in plankton samples. Most tubellarians are predatory on invertebrates smaller than themselves, while others are mainly herbivores, ectoparasites, or scavengers. Most species have organs of both sexes (hermaphrodite), although most have developed ways of avoiding self-fertilization. Copulation is often by injection of sperm directly through the epidermis of their mate. Eggs are deposited in capsules on the sea bottom.

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    Zoogeography

    Particularly shallow coastal waters.

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