The collar or mesosome is the middle ,short and cylindrical part. Its flap-like or funnel-like anterior margin,termed collarette, completely surrounds and conceals the proboscis stalk and the posterior part of proboscis.Ventrally, below mouth present.The wall of collar is thick,highly muscular and encloses a cavity, the collar coelom. It opens to outside through a pair of collar pores into the first pair of gill pouches behind.
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