Before leaving the topic of hydrostatic skeletons, a brief mention should be made concerning the molluscs, a group that you will study in some detail in the next lab. The molluscs are thought to be distantly related to the annelids (the two groups share a common early larval form, the trochophore) but the molluscs lack the segmentation found in the annelids. The molluscs also lack the spacious coelomic cavity found in the annelids. In the molluscs the coelomic space is taken up with reproductive and digestive organs. The open space in the molluscan body is actually a part of the circulatory system, which consists of a series of interconnected sinuses called the hemocoel. These sinuses, located in strategic regions of the molluscan body, become the cavity of the hydrostatic skeleton and blood serves as the hydrostatic fluid.

Obtain a live garden snail, Helix, and watch it for a moment. Notice how it extends its tentacles and eyestalks. How could this be accomplished? Gently probe the snail until it retreats into its shell, and watch it as it comes back out. How was the foot protruded? If you answered that blood is being pumped into the hemocoel sinuses and then used as hydrostatic fluid to work antagonistic muscle groups you have learned the lesson of hydrostatic skeletons very well. Congratulations!

(A) What is static about its hydrostatic system? - not much.

Actually many zoologists refer to the mollusc's use of blood and the hemocoel as a hydraulic system as opposed to a hydrostatic system. This is because blood is being transferred from one sinus to another rather than remaining in a single cavity. Think of the eyestalk being extended as analogous to a balloon being inflated. However once the blood has extended the extremity, the action of the antagonistic muscle groups is the same in both the hydrostatic and hydraulic systems.

Before completing this lab, be sure that you know the phyla and classes of organisms covered herein.