Invertebrate Chordates
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both
in the U.S. and worldwide. Using an open-content, web-based
collaborative model termed the FlexBook®, CK-12 intends to
pioneer the generation and distribution of high-quality educational
content that will serve both as core text as well as provide an
adaptive environment for learning, powered through the FlexBook
Platform®.
Copyright © 2014 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/terms.
Printed: November 2, 2014
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Invertebrate Chordates
1
Invertebrate Chordates
• Describe the two subphyla of invertebrate chordates.
• Distinguish between tunicates and lancelets.
Would you believe this animal eats its own brain?
This is a sea squirt, which is a tunicate. Many physical changes occur to the tunicate’s body during metamorphosis
into an adult, with one of the most interesting being the digestion of the cerebral ganglion, which controls movement
and is the equivalent of the human brain. From this comes the common saying that the sea squirt "eats its own brain."
Invertebrate Chordates
Living species of chordates are classified into three major subphyla: Vertebrata, Urochordata, and Cephalochordata.
Vertebrates are all chordates that have a backbone. The other two subphyla are invertebrate chordates that lack
a backbone. Members of the subphylum Urochordata are tunicates (also called sea squirts). Members of the
subphylum Cephalochordata are lancelets. Both tunicates and lancelets are small and primitive. They are probably
similar to the earliest chordates that evolved more than 500 million years ago.
Tunicates
There are about 3,000 living species of tunicates (see Figure 1.1). They inhabit shallow marine waters. Larval
tunicates are free-swimming. They have all four defining chordate traits (see the "Chordates" concept). Adult
tunicates are sessile. They no longer have a notochord or post-anal tail.
Adult tunicates are barrel-shaped. They have two openings that siphon water into and out of the body. The flow of
water provides food for filter feeding. Tunicates reproduce sexually. Each individual produces both male and female
gametes. However, they avoid self-fertilization. Tunicates can also reproduce asexually by budding.
1
www.ck12.org
FIGURE 1.1
Tunicates (Urochordata). Tunicates are
one of two subphyla of invertebrate chordates.
Lancelets
There are only about 25 living species of lancelets. They inhabit the ocean floor where the water is shallow. Lancelet
larvae are free-swimming. The adults can swim but spend most of their time buried in the sand. Like tunicates,
lancelets are filter feeders. They take in water through their mouth and expel it through an opening called the
atriopore (see Figure 1.2). Lancelets reproduce sexually and have separates sexes.
FIGURE 1.2
Lancelet (Cephalochordata). Unlike tunicates, lancelets retain all four defining chordate traits in the adult stage.
Can you find them?
Summary
• Chordates include vertebrates and invertebrates that have a notochord.
2
www.ck12.org
Chapter 1. Invertebrate Chordates
• Invertebrate chordates do not have a backbone.
• Invertebrate chordates include tunicates and lancelets. Both are primitive marine organisms.
Explore More
Use this resource to answer the questions that follow.
• Invertebrate members of the Phylum Chordata at http://www.nhc.ed.ac.uk/index.php?page=24.25.312.314
.
1.
2.
3.
4.
5.
6.
List three features of chordates.
How do invertebrate chordates differ from vertebrate chordates? Describe three differences.
How many species of invertebrate chordates have been identified?
What are the two subphyla of invertebrate chordates?
Describe the subphylum Urochordata. Give three characteristics.
Describe the subphylum Cephalochordata. Give three characteristics.
Review
1. Name and describe the two subphyla of invertebrate chordates.
References
1. Courtesy of US Geological Survey. Tunicates (Urochordata) are one of two subphyla of invertebrate chordates.
Public Domain
2. Hana Zavadska. Lancelet (Cephalochordata) illustration of body parts. CC BY-NC 3.0
3