Sprinkle, James Department of Geological Sciences, University of Texas, Austin, Texas.
- Links to Primary Literature
- Additional Readings
A small class of spindle-shaped, spirally pleated, primitive echinoderms in the subphylum Echinozoa, from the Early Cambrian (Nevadella Zone) in eastern California, western Nevada, and eastern British Columbia. Since their discovery in the early 1960s, three genera and six species have been described based on nearly 600 complete specimens, making helicoplacoids the most diverse and abundant echinoderm class known from the Early Cambrian. Most helicoplacoids have a spindle-shaped theca or body with diagonally spiraled pleats, each made up of three rows of flexibly sutured plates forming an interambulacral ridge. Smaller-plated ambulacra spiral around the theca every 7–13 pleats; the ambulacral plates cover a central food groove that has pores for tube feet on each side. The original authors inferred that the mouth was at the more rounded pole of the theca and that a single long ambulacrum branched once about one-third of the way down the theca. However, it has been argued that the mouth was located where the ambulacra split (see illustration), so that three ambulacral branches lead away from it, one spiraling partway down the theca and two spiraling up the theca a few pleats apart. No anal opening or pyramid has been found on any of the known specimens.
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 9000 highly qualified scientists and 43 Nobel Prize winners.
MORE THAN 8700 articles covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 19,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information