Hemichordata was earlier considered as a subphylum of Chordata but now it is placed as a separate phylum under non-Chordata.
Animals of this phylum are all fossorial.
Body worm like brittle and soft. Body is divided into three parts:
(i) Proboscis
(ii) Collar
(iii) Trunk
The body cavity is enterocoel0us i.e. divided into Protocoel, mesocoel, and metacoel.
Mostly ciliary feeders. Complete alimentary canal is present in digestive system.
Respiratory pigment vanadium is present in their blood. Respire by gill or body surface.
Circulatory system is open type. Blood is colourless with amoeboid corpuscles. Heart is situated dorsally.
True notochord is absent.
A notochord like structure is found in their buccal cavity, that is called “buccal diverticulum” or “stomochord” (Outgrowth structure of gut).
Post anal tail is absent.
Excretion is done by a single glomerulus. This single glomerulus is situated in the proboscis known as proboscis gland.
Central nervous system is just like non-chordates. Brain is present in the form of nerve ring.
Mostly animals are unisexual and reproduction is sexual.
Fertilization is external. Cleavage holoblastic.
Development is direct or indirect because some animals have tornaria larva just like the bipinnaria larva of Echinodermata in their development stage.
Hemichordata is divided into two classes
(i) Enteropneuta
e.g. (a) Balanoglossus – tongue worm or acorn worm.
(b) Saccoglossus
(c) Protoglossus
(ii) pterobranchia
e.g. (a) Rhabdopleura
(b) Cephalodiscus
Modern taxonomist does not include hemichordata in Chordata phylum. Hyman (1959) kept in separate phylum hemichordata in invertebrates.
Note: Hemichordata is connecting link between Non-chordata and Chordata.
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