On a New Type of Nephridia found in Indian Earthworms of the Genus Pheretima

Abstract

ABSTRACT There are three distinct kinds of nephridia in Phere-tima posthnma and other species of the genus, namely, the septal, the pharyngeal and the integumentary, named according to the position they occupy in the worm ; they differ from each other in size and also in respect of the place of opening of their ducts. Each nephridium is a separate and discrete structure and there is no network of any kind, and therefore the terms “plectonephric” and “diffuse” are inapplicable to these nephridia, although the term “micronephridia” can be retained. The integumentary nephridia are very minute and are hardly visible to the naked eye. Each of them has its own separate duct opening separately to the exterior on the skin. Each segment has about 200 to 250 of them. The pharyngeal nephridia occur in paired tufts lying at the sides of the œsophagus in the fourth, fifth and sixth segments; these nephridial tufts have three pairs of ducts, one in each segment, which open into the buccal cavity and the pharynx in the second and fourth segments. The septal nephridia are attached to both sides of the septa ; they do not open on the skin, but are connected with an elaborate system of ducts which ultimately open into the lumen of the intestine mid-dorsally at segmental intervals. The system of ducts and openings is perfectly segmental in arrangement, and consists of a pair of septal excretory canals on each septum which open into a pair of supra-intestinal excretory ducts lying in the mid-dorsal line of the worm above the dorsal wall of the gut, these ducts communicating with the lumen of the intestine at each intersegmental place by means of a narrow ductule. This last feature is characteristic of this type of nephridial system, and the latter has therefore been termed the “enteronephric” type of nephridial system. Each septal nephridium has a remarkable feature in that it lies—the funnel and all—wholly within the bounds of a single segment. The septal canals, the supra-intestinal ducts and their openings into the lumen of the gut have been described here for the first time, and the segmental character of the nephridial system of Pheretima, which was denied by Beddard, has now been established. The enteronephric nephridial system is probably a device for the conservation of water in this tropical genus, the gut-epithelium reabsorbing the water of the excretory fluid while letting the solid excretory matters pass out through the anus along with the fæces. Each nephridium in Pheretima, like that in Lumbricus, is connected with two blood-vessels, one bringing blood to it and the other taking away the blood. The septal and the integumentary nephridia are supplied with blood from the ventral vessel through the parietal vessel and the septal branch, and the blood is returned from these nephridia to the general blood-stream via the dorso-lateral vessel and the subneural vessel. The pharyngeal nephridia receive their blood-supply from the dorsal vessel, and the blood is returned from them to the lateral œsophageal vessels. The intersegmental septa in Pheretima posthuma and other species have definite, circular or oval apertures, each with a sphincter muscle around it. It is suggested that by closing these apertures by means of sphincters there is a restriction of the cœlomic fluid to certain segments which become consequently turgid and stiff, and are thus able to help the setæ in having a firm hold of the ground during the locomotory movements of the animal ; this turgidity of the part of the body applying setæ on to the ground seems necessary, since a bmp part cannot fix its setæ on the substratum on which the worm moves.