At places the oblique muscles are so oriented that they offer rigidity of the body by enhancing the hydrostatic pressure. It then bends its body like an inverted ‘U’ and drags the posterior sucker just behind the region of attachment. (iii) Male and female gonopores in the clitellum. This website includes study notes, research papers, essays, articles and other allied information submitted by visitors like YOU.
Segmented worms all display bilateral symmetry, cephalization, an open digestive system, segmentation, and a body cavity. The nephridia are ectodermal in origin. In Errantia each segment has a pair of nephridia. Body cavity of Phylum Annelida is a true coelom which lies between the two layers of mesoderm.
In larger forms of oligochaetes additional vessels like sub-intestinal or supra-intestinal vessel are often present. 3 weeks ago The continuity of the coelomic fluid in different body segment is caused through a foramen present in each transverse septum in the ventral nerve cord region. (i) Setae simple or forked, and may be 4 or 8 or sometimes multiplied in a ring in each segment.
Each solenocyte is a round cell with a slender tubular projection which anchors on the blind tube. The heart is a muscular tube which is an enlarged dorsal vessel with a number of lateral openings provided with valves, called ostia, through which blood enters into the heart from a large surrounding sinus, the pericardium.
17.60). Annelids have developed digestive systems, nervous systems and possess an enclosed circulatory system. 12. In most of the forms anterior, posterior, dorsal and ventral surfaces are well-recognised. In polychaetes these three types of muscles are less developed. A true blood- vascular system with close vessels and their ramification is present in the Acanthobdellida and the Rhynchobdellida. In Branchiura the last 40 segments are provided with thread-like, blood-filled evaginations. An excretory tubule which opens to the exterior through the nephridiopore and the inner end of the tubule is blind (associated with terminal cells or solenocytes) in the protonephridium or opens in the coelom through the ciliated funnel or called nephrostome in metanephridium. Development may be direct or indirect.
The remaining groups of polychaetes and others possess blood-vascular system and metanephridia. 1.
Asexual reproduction usually common in freshwater species and involves by the transverse division of the adult body. By the action of the segmentally arranged parapodia they paddle through water. Gas exchange takes place by diffusion through the moist body wall.
In sand or mud burrowers and tube-dwellers the parapodia are poorly developed or absent especially those of the posterior part of the body. Piscicola geometra is a brackish water form and remains attached to bottom- dwelling fishes. Fauvel (1953) reported that many of the polychaetes are really cosmopolitan and most of the species are common in the Indo-Pacific coasts. But recently the archiannelids are regarded to be the aberrant members of other annelidan families. 13. Development direct (e.g., Oligochaeta or Hirudinea) or indirect (e.g., Polychaeta, Archiannelida). Open into the enteric canal, e.g., Septal nephridia and pharyngeal nephridia. The nervous system, at least during development, exhibits a segmented condition, that is, its segmentation corresponds to the segmentation of the mesoderm. The whole body fossil record of annelids is scanty but well represented of semi-polychaetes have recorded from Middle Cambrian in Canada. Hirudo (the typical—leech which is parasite on vertebrates), Aulostoma (horse leech, free-living and carnivorous), Hirudinaria (H. granularia, cattle leech), Haemadipsa, Phytobdella (terrestrial leeches), Erpobdella (worm leeches), Dina. The circular muscles are thin and feebly developed.
As development proceeds, there occurs fusion of ganglia, the nerve cord and commissures.