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You are here:   OldClasses > 2012 > Turbellaria (within lobster mouth parts) | Brandon

 

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Turbellaria (within lobster mouth parts)                              

Brandon Meteyard (2012)

 

 

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Summary


Physical Description


Ecology


Life History & Behaviour


Anatomy & Physiology


Evolution & Systematics


Biogeographic Distribution


Conservation & Threats


References & Links

Evolution & Systematics

Riser & Morse (1974), came to the conclusion that there was an ancestral archetype (figure 1), containing the basic characteristics of every turbellarian order, similar to the theory of HAM seen in mollusca. The characteristics present are a one-layered epidermis, nervous system, eyes lacking or epidermal, weakly differentiated statocyst, simple mouth pore or pharynx, sac-shaped intestine, either no excretory organs or some kind of primitive protonephridia, female gonads (homocellular ovaries) and male genital pores. Riser & Morse (1974) believed throughout the phylogeny of platyhelminthes these characteristics were either lost or modified (Riser & Morse, 1974). 

Figure 1: Sagittal scheme of the Turbellarian archetype (Riser & Morse, 1974) 

Currently there are two theories which researches consider whilst discussing the phylogeny of turbellaria. The first is the planula theory, which states that turbellarians and cnidarians arose from a common ancestor, the planuloid, resembling a cnidarian larva (Ruppert et al., 2004). The second idea is that turbellarians evolved from a coelomate ancestor by anatomical simplification, the coelomate theory, and are not primitive to bilaterians (Ruppert et al., 2004). There are evidence for both theories and at current it is hard to work out exactly how they have come to evolve.

Morphological phylogentic tree of the Platyhelminth (Rupper et al., 2004)

1. Multiciliated epidermis and gastrodermis; 2. Mid-dorsal testes and anterior mid-dorsal male genopore; single mid-dorsal protonephridial terminal cell anterior in head, head well developed interstitial spaces or hemocoel; 3. concentration of gland cells is frontal organ; 4. protonephridia absent, ciliary rootlets interconnected forming fibrous skeletal network; 5. stratocyst bears a bilateral pair of statoliths, sperm has single flagellum; 6. stratocyst has one lens-shaped statolith and two cells lining the capsule; gland cells absent from gut; 7. duo-gland adhesive system, protonephridia has four cilia; 8. paired ventrolateral nerve cords joined post-orally; 9. sperm has two flagella each, muscular protrusible pharynx; 10. Large, dorsoventrally flattened body, extensively branched gut; 11. heterocellular ovary; 12. vitellocytes form an epithelial cortex over the oocyte; 13. sperm lack flagella; 14. original plicate pharynx modified into a muscular tube; 15. bulbous pharynx; 16. specialised bulbous pharynx located anteriorly with terminal opening. (Ruppert et al., 2004). 


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