CK-12 Foundation
Ck 12: Biology: Invertebrate Evolution
[Free Registration/Login may be required to access all resource tools.] An overview of the major events in invertebrate evolution.
BiologyWise
Biology Wise: Information About the Hydrostatic Skeleton
Explains what a hydrostatic skeleton is and the types of organisms that have one. Describes examples from six phyla and the advantages and disadvantages of this type of skeleton.