General Instructions for Dissection of ascaris (round worm)

While dissecting the animal keep in mind the following points.

  • Listen and follow carefully the instructions given by the teacher in your practical class.
  • Study well about the internal structures of the animal to be dissected.
  • Keep all the instruments in your dissecting box clean and sharp.
  • Always keep with you a Zoology Practical Book and also hand-drawn diagram of the dissection.
  • Remember that all invertebrates are dissected from dorsal side.
  • Wash the animal before dissection to remove excess of formaline or other fixing or killing chemical.
  • Keep a white sheet below the animal in dissecting dish.
  • Fix the animal in dissecting dish properly. Insert the pins obliquely.
  • While opening the animal never make deep incisions as in earthworm, etc.
  • Remove the body wall layers in such a manner that all the internal organs are fully exposed.
  • Keep your dissection submerged in water.

External features of nereis (clam worm)


For studying external feature take a formaline preserved animal. Fix it in a dissecting tray and study the following :

  1. Body form: Worm like. Body divided into head and similar 200 segments.
  2. Head: It consist of prostomium or pre-oral lobe and peristomium. Prostomium is the first segment and contains four eyes, two short prostomial tentacles and 2 large prostomial palps. Peristomium carries at each anterolateral position 4 pairs of peristomiai cirri. Peristomium contains mouth.
  3. Trunk: The main body is trunk made up of similar segment. Each segment contains a pair of parapodia.
  4. Anal segment : The last segment is devoid of parapodia and is called as anal segment having anus and a pair of anal cirri.

Digestive system Procedure

Take a preserved nereis wash it and fix in a dissecting tray. Make a superficial incision from anterior to posterior end, carefully remove the flap of skin and fix on side by pins. Study and draw the follwing parts :

  • Mouth: Transverse opening on the ventral side of prostomium. Mouth leads into buccal cavity.
  • Buccal cavity and pharynx : Buccal cavity leads into a muscular pharynx. The pharynx is everted forming proboscis. Internally pharynx contains jaws and denticles.
  • Oesophagus: The pharynx narrows posteriorly into oesophagus. The oesophagus contains a pair of oesophageal gland on side. Oesophagus opens into the stomach-intestine.
  • Stomach-intestine: It consists of segmentally constricted tube upto the last segment.
  • Rectum and anus: The stomach-intestine opens into rectum in pygidium. The rectum opens to the exterior through a terminal anus. Which has a pair of anal cerci.
  • Parapodium: Cut one or two parapodia of Nereis and Heteronereis from the side of the segment. Stain borax carmine, dehydrate, clear and mount in Canada balsam or D.P.X.
  • Other structures seen are protractor muscle, retractor muscle and nephridia meganephridia.

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