Circulatory System of Earthworm

Circulatory System

Blood-vascular system of earthworm is well developed and consists of closed blood vessels, which are longitudinal and transverse and in which red blood flows continuously.

The blood is bright red in colour due to the presence of haemoglobin or erythrocruorin which is dissolved in plasma. There are also corpuscles in the blood which are colourless and nucleated. RBCs are absent in earthworm.

The circulatory system is complicated and arrangement of blood vessels in the intestinal region i.e. behind the fourteenth segment is different from the arrangement in first fourteen

segments.

Fig. Blood-vascular system in first thirteen segments in Pheretima

The main longitudinal vessels are :

(i) Dorsal blood vessel,

(ii) Ventral blood vessel,

(iii) Lateral oesophageal vessels (paired),

(iv) Subneural vessel and,

(v) Supraoesophageal vessel (smallest).

Important transverse vessels in first 13 segments are:

Lateral hearts (Hearts pump blood from dorsal to ventral vessels, these have valves.)

Anterior loops

Lateral oesophageal hearts

Dorsal blood vessel is considered as true heart in earthworm. It is distributive in segments 1 to 13 and is collecting blood vessel in the segments 14 onwards. It collects blood from sub

neurals through commissural vessels. Flow of blood in dorsal vessel is from posterior direction.

Ventral vessel is found below alimentary canal, blood flows from anterior to posterior direction.

Ventral vessels give off paired tegumentary vessels in each segment to supply blood to segmental organs. After 13th segment, in each segment it sends paired ventro-intestinal vessels to intestinal wall.

Lateral oesophageal vessels are paired, located in the segments 1 to 13 on both sides of alimentary canal. After 13th segment, these unite to form a single sub-neural vessel.

Lateral oesophageal vessels collect blood from seminal vesicles present in 11^th and 12^th segments.

Sub-neural vessel runs from 14^th segment upto last segment, below the ventral nerve cord.

Lateral hearts : One pair in 7^th and one pair in 9th segment send blood from dorsal vessel to ventral vessel.

Lateral oesophageal hearts : One pair in 12^th and one pair in 13^th segment send blood from supra-oesophageal and dorsal vessel to ventral vessel.

Anterior loops : One pair in 10^th and one pair in 11^th segment, send blood from lateral oesophageal vessel to supraoesophageal vessel.

Blood glands are located in the segments 4, 5 and 6 above the pharyngeal mass.

Blood glands serve for the manufacture of blood corpuscles and haemoglobin (regarded as excretory by some authors).

Lymph glands are present on both sides of dorsal blood vessel from segment 26th and those behind it (one pair per segment, small and whitish). They are supposed to produce certain phagocytic cells.