Blue Crab Dissection Guide

(modified from

Part 1: External Anatomy. Turn the crab so it is oriented dorsal side up in the dissection pan. Observe your crab closely and draw the different external views in the boxes below. Draw any abnormalities or discolorations that you notice with your animal.

  1. External Anatomy- Dorsal View
  1. External Anatomy – Ventral View
  1. External Anatomy- Ventral View.

For this drawing you will zoom in on the abdomen of the crab,

pull the abdomen down (or open) and draw what you see.

Part II: Internal Anatomy:

Insert the tip of a heavy scissors beneath the lateral, posterior edge of the carapace and make a cut around the periphery of the carapace on its dorsal surface. Be careful that you cut only the heavy calcified exoskeleton and not the organs beneath it. Keep your scissors about 5 mm from the edge of the carapace and cut completely around it. Use a scalpel to separate it (by scraping, not cutting) from the underlying tissues. Carefully remove the carapace, in pieces if necessary, with minimal disturbance to the underlying tissues.

The thin, dark body wall, which is little more than the epidermis, lies immediately beneath the carapace and as much of it as possible should be removed with the carapace. Start to take note of the interior structures. Draw what you see in the box below. IMPORTANT TO DRAW BEFORE YOU MOVE ON WITH THE LAB!

If your specimen is a mature female, the orange ovaries may cover and obscure other structures. It may be necessary to remove the ovary (but nothing else) from one side in order to see the stomach and digestive ceca beneath. The smaller, white testes of the mature male do not obscure other structures. The two long, paired, indistinct, white or grayish, testes(convoluted tubes)may be difficult to distinguish from the digestive ceca beneath (ask your teacher for help).

The cardiacstomach is a large, bulging, transparent, thin-walled sac lying dorsally on the midline in the anterior thorax (GENTLY lift the posterior end of the cardiac stomach and look beneath it for the small pouchlikepyloric stomach. The digestive ceca (gland) are large, soft, amorphous, yellow or greenish organs occupying the periphery of the dorsal thorax. They may be completely obscured by the ovary in mature females

Locate the large, triangular, firm, beige or greyish mass of gills . Water flows in the inhalant aperture to the ventral inhalant chamber, then across the gill filaments into the dorsal exhalant chamber . It then exits via the exhalant aperture. Insert a blunt probe into the inhalant aperture at the base of the cheliped and observe that it enters the ventral inhalant chamber, below the gills. The probe can be pushed gently upward through the curtain of gills into the dorsal exhalant chamber above the gills thus tracing the route taken by the respiratory water current through the gill chamber (ask your teacher for help if you cannot find these chambers). The gills are sometimes called "dead man's fingers".

The triangular mass of gills is covered by a very thin, transparent membrane which you should avoid damaging. The soft, white or gray heart lies on the midline posterior to the stomach and next to the walking leg muscles.

Pre-lab questions: Print out/fill-in or write out the answers to these questions and bring to class on the day of the dissection.

  1. What is the first thing you are going to do in the lab?
  1. How many structures are you going to have to locate ,draw and label for the external dorsal view?______
  1. What do the lines mean next to the name of the structures in the draw and label box?
  1. Explain what you will do for the external anatomy-ventral view drawing?
  1. Label the structures for this part requires that you know the ______of your crab.
  1. Explain how you cut open your crab and draw arrows on the crab below showing where you start cutting the carapace and the direction in which you cut.

  1. What do you have to draw for the internal anatomy of the crab?
  1. What do you think those little boxes are meant for that are next to the bolded terms or structures in the procedure?