In this article we will discuss about the external morphology of roundworm or Ascaris.
Ascaris resembles an ordinary earthworm and is the largest intestinal nematode parasitising man. It is milk-white in colour but presents a reddish-yellow shade. Body is elongated, cylindrical, and gradually tapering at both ends. The females are usually 20-25 cm in length and 5 mm in diameter while the males are 15-17 cm in length and 3 mm in diameter.
Female worms are larger than the males and are further distinguished by the presence of separate and independent genital aperture or vulva on the ventral surface at about one-third of the body’s total length from the anterior end. The post anal portion in the male is sharply curved downwards while in the female it is nearly straight.
Tail end of male is characterised by the presence of numerous genital papillae on the ventral surface. There are about 50 pairs of pre-anal papillae and 5 pairs of post-anal papillae. These genital papillae of male help in copulation. While in female only one pair caudal papillae are present.
A triangular mouth is situated at the anterior end and is bounded by three lips, one median and dorsal, and two ventro-lateral in position. Each lip is provided with denticles in its inner margin and sensory papillae on the outer margin. In male the anus is replaced by cloaca which is situated posteriorly about 2 mm from the posterior end on the ventral surface. It is the common aperture for digestive and genital tubes. In male a pair of needle like chitinoid bodies project from this aperture which are known as penial setae or spicules.
These spicules serve to transfer sperms into female vagina during copulation. In male a little down the anterior tip and on the ventral surface is the excretory pore. The female has an anus a little in front of the posterior end on the ventral surface. Only the digestive tube opens to outside through anus.
The body is marked with four longitudinal streaks or lines running along the entire length of the body. Of these four streaks, one is dorsal, one is ventral and two are lateral in position (Fig. 11.1).