Leeches are annelids that constitute the subclass Hirudinea. Almost all leeches are ectoparasites that attach to the host’s external surface. Like most other annelids, they also have an open circulatory system. Here, let’s learn more about the circulatory system of leeches.
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Leech – General Description
Most leeches are freshwater, but some are marine or terrestrial in nature. They are ectoparasitic, blood-sucking, or carnivorous in general. They are hermaphrodites showing sexual reproduction.
Their body is more elongated, cylindrical or dorso-ventrally flattened. The body has no setae or parapodia. Most leeches have suckers at both the posterior and anterior ends. They also have 33 segmentations (somites), and these segmentations are concealed by external ring-like markings called annulations. The annuli are divided by transverse separation walls called septa. Also the number of annuli varies in each segment of the leech. Annular receptors can be found in each annulus, while segmental receptors can be found on the 1st annulus of each segment. Each annelid segment contains nervous, excretory, and circulatory system units.
Also Check: Diagram of Leech (Hirudinaria granulosa)
Leech Circulatory System
Leeches show an open circulatory system. They transport gases across their body walls. Some leeches retain the basic circulatory pattern of Annelida, but it is highly modified in most leeches, with coelomic channels replacing vessels. Coelomic fluid has replaced blood, and respiratory pigments are absent except in two orders.
The haemocoelomic system of leeches is made up of coelomic channels. Coelomic channels are filled with a type of blood-like fluid and these channels represent the blood vessels. Haemoglobin is found in coelomic fluid. These channels are used to circulate colourless corpuscles. There are four such channels that make up the haemocoelomic system:
- One ventral channel – It runs mid-ventrally below the alimentary canal from one end to the other end of the body. Here, the fluid flows from the anterior to the posterior end.
- One dorsal channel – It runs mid-dorsally above the alimentary canal. Both dorsal and ventral channels are devoid of valves.
- Two lateral channels – These 2 channels are present on either side of the leech’s alimentary canal. They are valvular and muscular. In the dorsal and 2 lateral channels, the haemocoelomic fluid flows from behind to forward.
All these 4 channels are posteriorly connected together at the 26th segment.
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