Biol 212 Zoology
Lab 11: The Invertebrate Chordates (10 points)
Introduction
The phylum Chordata is the phylum to which we belong. Sharing the phylum are cats,
dogs and other mammals; reptiles and birds; amphibians and fishes of all sorts; and a couple of
groups of animals that are generally not familiar to most people, unless you are an invertebrate
zoologist!
Chordates are bilateral, triploblastic deuterostomes with a well-developed, enterocoelic
coelom. Besides these characteristics, there are five synapomorphic characteristics that unite the
three subphyla within the phylum Chordata. These five characteristics are as follows:
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Notochord. The notochord is a structure developed from the mesoderm that extends the
length of the body and provides support and resistance against which muscles can work,
allowing the animal to move. All chordates have the notochord during early development,
and more primitive chordates retain it into adulthood; in advance chordates, the notochord is
absorbed into the structure of the backbone.
Dorsal, hollow nerve cord. The dorsal, hollow nerve cord is a singular, hollow tube that runs
superior to the gut along the dorsal aspect of the body; in the anterior region, it generally
widens and develops into a brain, still maintaining hollow areas called vesicles. The dorsal,
hollow nerve cord forms the central structure of the nervous system. In non-chordate
animals, the nervous system is ventral, solid and sometimes consists of more than one nerve
cord. In vertebrates, vertebrae develop around the dorsal, hollow nerve cord, protecting it in a
sheath of cartilage or bone.
Pharyngeal gill slits. The pharyngeal gill slits are formed from the external invagination of
ectoderm and the internal evagination of endoderm. In aquatic chordates, these structures
merge together, forming gill slits. In higher chordates, the gill slits do not completely form;
rather, the internal evaginations develop into the auditory (Eustachian) tube and middle ear,
tonsils and parathyroid glands.
Dorsal hollow nerve cord
Notochord
Endostyle
Pharyngeal gill slits
Postanal tail
Fig. 11.1: Diagrammatic representation of an early chordate showing
synapomorphic characteristics uniting the phylum.
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Endostyle. The endostyle is a structure found in primitive chordates and some primitive fish
larvae. The endostyle secretes mucus, helping to trap food particles during feeding; some
cells within the endostyle produce iodinated protein molecules. In more advanced chordates,
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the endostyle develops into the thyroid gland, which secretes the hormone thyroxine, an
iodinated protein molecule. All chordates have a thyroid gland, needed to regulate
metabolism.
Postanal tail. The postanal tail, which extends beyond the anus, is present at some time
during development of all chordates. The postanal tail provides an enhanced propulsion
capability for aquatic chordates such as amphibians, fishes, reptiles and some mammals, and
has evolved into an organ with other functions as seen in monkeys, kangaroos, dogs and cats.
There are three subphyla that make up the phylum Chordata: the Urochordata (=
Tunicata), Cephalochordata and Vertebrata. Because of the importance of this phylum, there will
be several labs in this course giving you the opportunity to study the chordates.
The purpose of this lab is to introduce you to the invertebrate chordates.
For the Lab Report:
*On the upper, right-hand corner of your lab report, print your name, Biol 212, Lab 11: The
Invertebrate Chordates, and the date you did this lab.
Introductory Objectives
Objective 1: Be able to list and describe the five synapomorphies that unite the phylum Chordata.
Objective 2: List the three subphyla that make up the phylum Chordata.
For the Lab Report:
Write out these questions then answer them:
1. List and describe the five synapomorphies that unite the phylum Chordata.
2. Give the three subphyla that make up the phylum Chordata.
Exercise 11.1: Subphylum Urochordata
Introduction
The members of the subphylum Urochordata, also called “tunicates” or “sea squirts,” are
strictly marine. Adult tunicates are sessile, attached to hard substrates, and may be solitary or
colonial. They have an outer coat called the tunic, which is made of proteins and carbohydrates,
including very high amounts of cellulose; the tunic is a non-living secretion.
Extending from the tunic are the incurrent (oral) and excurrent (atrial) siphons (see Fig.
11.2). Ciliary currents bring water into the animal via the incurrent siphon. The water passes
through the mouth and into the pharynx where it is strained through slits in the pharyngeal
basket. The endostyle, located mid ventrally in the pharyngeal basket, secretes a sheet of
mucous, which is moved by cilia across the internal surface of the pharyngeal basket to the
dorsal side. The mucous sheet traps food particles and forms a mucous rope as it is moved into
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the esophagus. The water that has just been
filtered through the pharyngeal basket enters the
cloacal chamber and is expelled through the
excurrent (atrial) siphon. Food that has been
taken in is transported from the esophagus to
the stomach, digested in the stomach and
intestine, absorbed by the intestine, with
material that was not digested or absorbed being
expelled into the cloacal chamber from the
anus. The gut is characteristically and
distinctively “U-“ shaped, which one can see
through the gelatinous tunics of some species of
solitary and compound tinicates.
The circulatory system of tunicates
consists of a simple heart with vessels extending
from either direction into hemal sinuses. Of note, the
Fig. 11.2: Tunicate adult anatomy.
heart will periodically stop then restart, pumping blood in the opposite direction; among
chordates, this phenomenon is only seen in tunicates.
Most tunicates lack excretory organs. Tunicates are hermaphrodites, and often contain
both a single ovary and testis.
It is interesting to note that adult urochordates only exhibit two of the five characteristics
unique to chordates,
the pharyngeal gill
slits and an
endostyle. However,
if we look at
urochordate larvae,
called “tadpole
larvae,” all five of
the hallmark
characteristics are
found.
Fig. 11.3: Tunicate tadpole larva.
Urochordate tadpole larvae
superficially resemble frog tadpoles
in that the body is a rounded mass, followed by an elongate, muscular, post-anal tail (see Fig.
11.3). The tail allows the tadpole larvae to be free-swimming, unlike the
adult form. Like the adult, the tadpole larva has a pharyngeal basket (sac) with pharyngeal gill
slits and an endostyle. Further, there is an incurrent (branchial) and excurrent (atrial) siphon, but
since the tadpole larva is a transient stage that does not feed, these structures are blocked. There
are also a dorsal, hollow nerve cord and notochord located beneath that structure. Other structure
seen in the adult including the developing digestive system and functioning heart.
The tadpole larva may persist only a few minutes to two days, depending on the species.
During this time, the tadpole will chemically detect a suitable substrate, attach using its adhesive
papilla, then metamorphose into the sessile adult, as in Fig. 11.4. An interesting note is that
during this development, the nervous system, including the brain, is markedly reduced.
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Objectives
Objective 3: Identify the
following structures in a
plastomount, preserved
specimen, microscope slide
or photograph of a solitary
tunicate: tunic, incurrent
(oral) siphon, mouth,
pharyngeal basket,
endostyle, esophagus,
stomach, intestine, cloacal
Fig. 11.4: Metamorphosis of a tunicate. Ex.
chamber, excurrent (atrial) siphon,
Brusca and Brusca.
heart, gonad and genital duct.
Objective 4: Identify a tunicate
tadpole larva from a microscope slide or photograph; identify the following structures in a
microscope slide or photograph of a tunicate tadpole larva: adhesive papillus, post-anal tail,
incurrent (oral) siphon, pharyngeal basket, excurrent (atrial) siphon, dorsal hollow nerve cord
and notochord.
Objective 5: From preserved specimens or photographs, identify solitary and colonial
representatives of the chordate subphylum Urochordata to phylum and subphylum.
Materials and Methods
*Plastomount (anatomical preparation) or preserved representative or microscope slide of adult
tunicates (demo)
*Microscope slide of urochordate (ascidian/tunicate) tadpole larva
*Plastomount or preserved solitary and compound tunicates (demo)
-Dissecting and compound microscopes
1. Examine the external anatomy of a representative adult using either a dissecting microscope,
if the specimen is in a plastomount, or a compound microscope, if the specimen is on a
microscope slide. Identify, draw and label the structures indicated as per the For the Lab
Report box below.
For the Lab Report:
3. Write out, “3. Phylum Chordata, Subphylum Urochordata, external anatomy of representative
tunicate [or Genus species, if known].” Identify, draw and label the tunic, incurrent (oral)
siphon and excurrent (atrial) siphon. Include an accurate size rule next to your drawing. No
credit for drawings without accurate size rules. To the right of your drawing, include the size
of the size rule Also, include any notes that might help you to identify the organism on the
lab practical! Have your instructor check and initial your drawings for credit; all drawings
must be completed in lab and signed by your instructor for credit!
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2. Examine the internal anatomy of a representative adult tunicate. Identify, draw and label the
structures indicated as per the For the Lab Report box below.
For the Lab Report:
4. Write out, “4. Phylum Chordata, Subphylum Urochordata, internal anatomy of representative
tunicate [or Genus species, if known].” Identify, draw and label the tunic, incurrent (oral)
siphon, mouth, pharyngeal basket, endostyle, esophagus, stomach, intestine, cloacal chamber,
excurrent (atrial) siphon, heart, gonad and genital duct. Include an accurate size rule next to
your drawing. No credit for drawings without accurate size rules. To the right of your
drawing, include the size of the size rule Also, include any notes that might help you to
identify the organism on the lab practical! Have your instructor check and initial your
drawings for credit; all drawings must be completed in lab and signed by your instructor for
credit!
4. Obtain a slide of a tunicate tadpole larva. Draw an overall sketch of the animal, with a size
rule. Identify, draw and label the structures indicated as per the For the Lab Report box
below.
For the Lab Report:
5. Write out, “5. Phylum Chordata, Subphylum Urochordata, anatomy of representative
urochordate tadpole larva [include Genus species, if known].” Identify, draw and label the
adhesive papillae, post-anal tail, incurrent (oral) siphon, pharyngeal basket, excurrent (atrial)
siphon, dorsal hollow nerve cord and notochord. Include an accurate size rule next to your
drawing. No credit for drawings without accurate size rules. To the right of your drawing,
include the size of the size rule Also, include any notes that might help you to identify the
organism on the lab practical! Have your instructor check and initial your drawings for
credit; all drawings must be completed in lab and signed by your instructor for credit!
5. Examine any solitary or colonial tunicates on display, or photographs of tunicates. Suggest,
as per the For the Lab Report box below, characteristics you could use in the field to identify
tunicates.
For the Lab Report:
Write out these questions then answer them:
6. What characteristics could be used to identify living tunicates in the field?
Exercise 11.2: Subphylum Cephalochordata
Introduction
Members of the subphylum Cephalochordata are also called lancets or amphioxus, as
Amphioxus was an old genus name that is still used as a common name. Lancets superficially
look like little, transparent fish, up to about 7 cm in length, that bury themselves in marine
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Fig. 11.5: Internal anatomy of a lancet.
sediments with their anterior ends protruding out.
Although at first glance they resemble fish, they lack all
of the fins that true fish have, except for a caudal (tail) fin
and a bit of dorsal fin. They also don’t have a vertebral
column, jaws, a true head (cranium).
Both larval and adult lancets possess all five
synapomorphies that unite members of the phylum
Chordata together: notochord, dorsal hollow nerve
cord, pharyngeal gill slits, an endostyle and postanal tail.
Overall, the notochord, dorsal hollow nerve cord, post
anal tail and pharynx with slits and endostyle are fairly
obvious. Note also that the musculature of the lancet is
arranged in distinct chevrons or “V” forms, very much
like a fish.
The digestive system of the lancet is complete and
begins with an oral hood with tentacles surrounding the
mouth and located under the rostrum (nose). A water
current is established by cilia in the buccal (mouth)
cavity. Water is brought in through the oral hood into the
pharynx, through the more than 100 pharyngeal gill
slits, where food is trapped in mucus produced by the
Fig. 11.6: Transverse section showing
endostyle. Water leaves via the atrium then out
the internal anatomy of a lancet.
through the atriopore. Food and mucous are moved
into a stomach-like area that receives digestive enzymes
from the digestive cecum. From here, nutrients enter the intestine for absorption, then materials
that were not digested or absorbed are voided through the anus.
The circulatory system of the lancet is closed, very similar in pattern to vertebrates,
though there is no heart. The walls of the ventral aorta pulsate, pushing blood forward and up
through the vessels of the pharyngeal gill slits, where carbon dioxide is released into the water
and oxygen is picked up. From there, blood enters the dorsal aorta and is moved posteriorly
through intestinal capillaries, where nutrients are picked up, and to the myotomes (muscles).
Blood then drains from the posterior through the cecum, in a manner similar to blood going to
the liver in vertebrates, then into the ventral aorta.
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The genus commonly studied in biology laboratories is Branchistoma, formerly
Amphioxus.
Objectives
Objective 6: Identify the following structures in a preserved Branchiostoma (Amphioxis):
rostrum, eyespot, dorsal hollow nerve cord, notochord, post anal tail (caudal fin), oral hood,
pharynx, gill slits, cecum, metapleural fold, atrium, atriopore, stomach, intestine, anus.
Objective 7: Identify the following structures in a cross section of the pharynx of a
Branchiostoma (Amphioxus): epidermis, dorsal fin, dorsal hollow nerve cord, notochord,
myotomes, pharynx, endostyle, coelom, atrium, gill bars and slits, ventral aorta, dorsal aorta,
cecum, gonad.
Objective 8: Identify a representative of the genus Branchiostoma (Amphioxis) as to phylum,
subphylum and genus.
Materials and Methods
-Dissecting and compound microscope
*Preserved whole Branchiostoma (Amphioxis) (demo; not for dissection)
-Watchglasses
-Probes, forceps for manipulating preserved Branchiostoma (Amphioxis) in watchglasses
*Microscope slide of Branchiostoma (Amphioxis) (cs), stained
1. Place a preserved Branchiostoma (Amphioxis) under a dissecting microscope. Draw it,
identify and label it as per the For the Lab Report box below.
For the Lab Report:
7. Write out, “7. Phylum Chordata, Subphylum Cephalochordata, anatomy of Branchiostoma.”
Sketch the entire animal, then identify, draw and label the rostrum, eyespot, dorsal hollow
nerve cord, notochord, post anal tail (caudal fin), oral hood, pharyngeal basket (pharynx), gill
slits, cecum, metapleural fold, atrium, atriopore, stomach, intestine and anus. Include an
accurate size rule next to your drawing. No credit for drawings without accurate size rules.
To the right of your drawing, include the size of the size rule Also, include any notes that
might help you to identify the organism on the lab practical! Have your instructor check and
initial your drawings for credit; all drawings must be completed in lab and signed by your
instructor for credit!
2. Examine a cross section (cs) of a Branchiostoma (Amphioxis) pharynx under a compound
microscope. Draw it, identify and label it as indicated in the For the Lab Report box below.
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For the Lab Report:
8. Write out, “8. Phylum Chordata, Subphylum Cephalochordata, cross section of
Branchiostoma.” Sketch the cross section, then identify, draw and label the epidermis,
epidermis, dorsal fin, dorsal hollow nerve cord, notochord, myotomes, pharyngeal basket,
coelomic cavity, atrial cavity (atrium), gill slits, gill bars, endostyle, ventral aorta, hepatic
cecum, gonad.. Include an accurate size rule next to your drawing. No credit for drawings
without accurate size rules. To the right of your drawing, include the size of the size rule
Also, include any notes that might help you to identify the organism on the lab practical!
Have your instructor check and initial your drawings for credit; all drawings must be
completed in lab and signed by your instructor for credit!
For the Lab Report:
Write out these questions then answer them:
9. What characteristics do the urochordates and cephalochordates have in common with the
subphylum Vertebrata?
10. What characteristics do the vertebrates have that the urochordates and cephalochordates
lack?
~When you’re finished, help clean up!
1. Is your lab bench clean and wiped down with antiseptic solution?
2. Are all materials returned to their proper place?
3. Is the oil immersion objective of your microscope clean?
4. Is the lowest-power objective of your microscope positioned down?
5. Is the power cord draped loosely about one of the oculars?
6. Is your microscope put away?
7. Is all refuse disposed of properly?
8. Is the lab generally in order?
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