Peristomium and Prostomium
The first segment of the earthworm’s body is peristomium, which bears a small crescent shaped mouth at its terminal end. A small tongue-like lobe of skin called as prostomium, projects out from the first body segment. Prostomium varies in size and shape in some worms. It may be so small that it cannot be distinguishable. The junction of the prostomium with peristomium differs in species, and is of systematic importance.
Fig: Scanning microphotograph of mouth in Eisenia foetida
(i) Prolobic: When a separating groove not extending to the first segment mark, it is called Prolobic.
(ii) Epilobic: When it extends more markedly on the first segment, called as epilobous. The extension or elongation into the region of the first segment is called tongue, it may be limited behind by the transverse groove or there may be no such groove. Depending on the length of the tongue whether it is one third or half or two third length of first segment, the prostomium is described as prostomium epilobic 1/3, 1/2 or 1/3.
(iii) Tanylobic: If the tongue touches the groove between i and ii se gment it is called tanylobic
(iv) Zygolobic: If the prostomium is not marked off by a groove from the segment, it is zygolobic.
Fig: Diagrammatic sketch of various forms of Prostomium
Fig: Various forms of Prostomium: A. Prolobic; B. Epilobic; C. Tanylobic; D. Zygolobic