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Phylum Hemichordata: Characteristics, Classification
Phylum Hemichordata (Gr. hemi – half)
1. Body is soft, unsegmented worm-like, and bilaterally symmetrical. These animals possess a combination of invertebrate (non-chordate) and chordate characters.
2. Body is divided into proboscis, collar (collarette), and trunk.
3. These animals resemble chordates only in having pharyngeal gill clefts or gill slits and they lack notochord and true dorsal nerve cord. Gill slits form the respiratory system and connect the pharynx with the outside.
4. ‘Buccal diverticulum’, earlier regarded as “notochord” is present in the proboscis.
5. No nephridia. Single glomerulus connected to blood vessels has an excretory function.
6. Exclusively marine, solitary, or colonial, mostly tubicolous and detritus feeders like the earthworms.
Examples: Balanoglossus (acorn worm or tongue worm), Cephalodiscus.
Differences between non-chordates and chordates
|1. Notochord is present in them at some of their developmental stages.||1. Notochord is present in them at some of their developmental stages.|
|2. Their central nervous system is hollow and dorsal.||2. Their central nervous system is hollow and dorsal.|
|3. Heart is always present and it is ventral in position.||3. Heart is always present and it is ventral in position.|
|4. Their circulatory system is of a closed type.||4. Their circulatory system is of dosed type.|
|5. Haemoglobin is present in their red blood cells or corpuscles.||5. Hacmoglobin is present in their red blood cells or corpuscles.|
|6. Pharyngeal gill slits are present in them.||6. Pharyngeal gill slits are present in them.|
|7. Their anus is not located at the posterior tip of the body, so contains a post-anal tail.||7. Their anus is not located at the posterior tip of the body, so contains a postanal tail.|