In this article we will discuss about of Polygordius:- 1. External Structures of Polygordius 2. Body Wall and Coelom of Polygordius 3. Digestive System 4. Circulatory System 5. Excretory System 6. Nervous System 7. Reproductive System.
External Structures of Polygordius:
The body of polygordius is slender, elongated, cylindrical and measures about 2-10 cm in length (Fig. 17.41). The segmentations are poorly marked, specially at the anterior part. Numerous identical segments are marked at the posterior part. The prostomium contains a pair of tentacles. The dorso-lateral side of the body behind prostomium bears a pair of ciliated grooves.
The peristomium is more developed than the prostomium. Mouth is ventrally placed near the junction of prostomium and peristomium. The posterior- most segment which contains anus, is broad and conspicuous. A series of tooth-like processes are present round the anus and a circlet of adhesive papillae lie in front of it. The para-podia are absent and setae are rarely seen.
Body Wall and Coelom of Polygordius:
The body wall of of Polygordius resembles that of other annelids but the layers of circular muscles are absent.
Coelom:
The body cavity or coelom is largely obliterated. The septa are well-marked and complete.
Digestive System of Polygordius:
The mouth of of Polygordius leads into an alimentary canal which includes short, slightly protrusible buccal chamber, oesophagus, stomach and intestine. The entire length of the canal is internally lined by cilia. The anus becomes conspicuous due to the presence of tooth-like processes and adhesive papillae. The animals feed on small algae.
Circulatory System of Polygordius:
The circulatory system of of Polygordius is represented only by a dorsal and a ventral vessel which run longitudinally. These two vessels are connected by several transverse or lateral vessels.
Excretory System of Polygordius:
The system includes a series of metamerically arranged nephridia, excepting in the first and last segments. The internal opening of each nephridium is situated on the proceeding segment and its external opening is called nephridiopore.
Nervous System of Polygordius:
A pair of cerebral ganglia in the prostomium and a ventral nerve cord without segmental ganglia, represent the nervous system. The ventral nerve cord is embedded within the epidermis. The paired ciliated grooves which are present behind the prostomium probably act as sense organs.
Reproductive System of Polygordius:
Sexes of of Polygordius are separate. The females are usually larger than the males. Sex organs and ducts are not well recognised. Testis or ovary develops from the coelomic epithelium and the sex cells are discharged in the body cavity. In both the sexes, the reproductive cells are liberated by the rupturing of the body wall.
Fertilization is external in of Polygordius. The egg contains little yolk and the incidents of early development resemble that of Nereis. Here instead of epibolic movement of the micromeres the macromeres invaginate. The larva which is also called trochophore (Fig. 17.41) resembles in all respects the trochophore of Nereis. It is spherical and filled up with body fluid.
The adult worm is formed by posterior elongation and segmentation of the anal end of the trochophore. Each segment possesses a part of gut with enveloping mesoderm and ectoderm. The mesoderm in these segments splits to give rise to coelom. The body gradually elongates and the trochophore shape remains as swollen ‘head’ of the metamorphosing larva. This end gradually transforms into the head of the adult.
The life history of Polygordius thus exhibits a scheme whereby a triploblastic coelomate organism may be evolved from a simple diploblastic gastrula; it also speaks about the origin of metameric segmentation.
Members of the class Archiannelida are considered as ancestral forms from which larger groups of annelids have been evolved. Addition of parapodia and setae and removal of ciliation and the connection be tween nervous system and epidermis in an archiarmelid converts it to a chaetopod. Such resemblance speaks about their phylogenetic interrelationship.