Protochordates
Summary
TLDRThis educational video explores the lesser-known but biologically significant group of proto chordates, which includes sea squirts (tunicates) and lancelets. Both are part of the phylum Chordata, sharing key characteristics like a notochord and dorsal nerve cord. The video delves into their anatomy, highlighting the lancelet's segmented muscles and the tunicate's filter-feeding system. It also discusses the new taxonomic classification that separates them into subphyla, reflecting recent molecular evidence.
Takeaways
- 🐠 Protochordates, including lancelets and tunicates, are crucial for understanding the early evolution of the phylum Chordata, to which humans belong.
- 📚 Both lancelets and tunicates are part of the invertebrate chordates and are studied in the invertebrates 3 laboratory exercise.
- 🔍 All chordates share four key characteristics at some point in their life cycle: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a muscular post-anal tail.
- 🏷️ Protochordates are classified into two subphyla, reflecting a new taxonomic category between the phylum and class levels.
- 🧬 Molecular evidence has led to revisions in the classification system, distinguishing lancelets in the subphylum Cephalochordata and tunicates in the subphylum Urochordata.
- 🔬 Lancelets, or amphioxus, have a simple body structure with a dorsal fin, caudal fins, and a segmented body resembling early chordates and annelids.
- 👀 Lancelets lack eyes but have simple eye spots on the nerve cord for light detection, and they do not have paired appendages like pectoral or pelvic fins.
- 🍽️ Lancelets are filter feeders, using cilia and mucus to capture microscopic organisms and transport them to the stomach for digestion.
- 🌊 The pharynx of lancelets has gill slits that allow water to pass through, facilitating respiration and the filtration of food.
- 💊 The hepatic cecum in lancelets is believed to be homologous to the liver in vertebrates, playing a role in digestion and nutrient absorption.
- 🔄 Tunicates, as adults, are sessile and filter feed, losing certain chordate characteristics, but their larval form exhibits all the defining features of the phylum.
Q & A
What are the two groups of organisms referred to as proto chordates or invertebrate chordates?
-The two groups of organisms referred to as proto chordates or invertebrate chordates are sea squirts, also known as tunicates, and lancelets.
Why are proto chordates important in biology?
-Proto chordates are important in biology because they help us understand the early evolution of the phylum Chordata, to which vertebrates, including humans, belong.
What are the four derived characters shared by all chordates at some point in their life cycle?
-The four derived characters shared by all chordates at some point in their life cycle are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a muscular post-anal tail.
What is the difference between the subphylum Cephalochordata and Urochordata?
-Cephalochordata is the subphylum that includes lancelets, while Urochordata includes tunicates or sea squirts. Both are part of the phylum Chordata but have different characteristics and life cycles.
Why did taxonomists revise the classification system for chordates?
-Taxonomists revised the classification system for chordates due to molecular evidence that suggested all groups should be monophyletic, leading to a reevaluation of the relationships among chordate groups.
What is the common name for lancelets and why is it called that?
-The common name for lancelets is 'lens' or 'lentils', due to their overall shape, which is similar to the shape of a lentil, and the no longer used genus name amphioxus, meaning pointed at both ends.
What are the structures known as myomeres in lancelets?
-Myomeres, also known as segmental muscles, are visible through the thin epidermis of the lancelet. They are arranged similarly to that seen in fish and provide evidence that the earliest chordates had a segmented body.
How do lancelets feed and what is the role of the wheel organ in this process?
-Lancelets are filter feeders that depend on cilia to draw water and food into their digestive system. The wheel organ, located at the front of the velum, along with the inner surface of the pharynx, is covered in cilia that help create a current to draw in microscopic organisms.
What is the function of the hepatic cecum in lancelets and how is it related to the liver in vertebrates?
-The hepatic cecum in lancelets is a blind-ended pouch that secretes enzymes and takes part in the absorption of nutrients. It is considered homologous with the liver in vertebrates, indicating a shared evolutionary origin.
How do adult tunicates differ from their larval form in terms of the presence of chordate characters?
-Adult tunicates, being sessile, lose certain structures, including three of the four derived characters of the phylum Chordata. However, their larval form reveals all four expected characters, indicating their chordate nature.
What is the role of the pharynx in the feeding process of adult tunicates?
-In adult tunicates, the pharynx, which occupies most of the body, functions as a filter-feeding apparatus. Water is drawn in through the incurrent siphon, and as it passes through the pharynx, planktonic creatures adhere to a mucus coating, which is then transported to the digestive system.
Outlines
🐚 Introduction to Protochordates
The script introduces two groups of organisms, the sea squirts (tunicates) and lancelets, collectively known as protochordates or invertebrate chordates. These organisms, though not well-known, play a crucial role in understanding the early evolution of the phylum Chordata, which includes vertebrates. Both groups exhibit key characteristics of chordates during some stage of their life cycle, such as a notochord, dorsal nerve cord, pharyngeal slits, and a post-anal tail. The script discusses the new classification system that places lancelets in the subphylum Cephalochordata and tunicates in the subphylum Urochordata. It also provides resources for further study, including textbooks and laboratory models, and describes the external and internal anatomy of lancelets, highlighting features that support their evolutionary significance.
🌊 Lancelets' Anatomy and Digestive System
This paragraph delves deeper into the anatomy of lancelets, focusing on their digestive system. It explains how lancelets, lacking jaws and a muscular pharynx, rely on cilia to draw in water and food. The script describes the structures of the buccal cavity, velum, pharynx, and the wheel organ, which are all involved in the feeding process. It also details the role of the atrium, a water chamber surrounding the pharynx, and the esophagus, stomach, intestine, and anus in the digestion and expulsion of waste. The paragraph further explores the internal organs, including the hepatic cecum, which is believed to be homologous with the liver in vertebrates, and the gonads. A cross-section of a lancelet is used to illustrate the positions of these organs relative to the notochord and nerve cord.
🌊 Tunicates: Adult and Larval Forms
The final paragraph shifts focus to tunicates, another group of protochordates. It clarifies why tunicates, appearing to lack some chordate characteristics in their adult form, are still classified within the phylum Chordata by examining their larval stage, which displays all the defining features. The script describes the adult tunicate's filter-feeding mechanism, involving the incurrent and excurrent siphons and the pharynx's role in capturing plankton. It also explains how the food-laden mucus is transported to the digestive system and how waste is expelled. The paragraph concludes by mentioning the availability of a hole mount slide of a tunicate in the laboratory for further study, thus rounding off the overview of protochordate specimens.
Mindmap
Keywords
💡Proto chordates
💡Tunicates
💡Lancelets
💡Notochord
💡Dorsal hollow nerve cord
💡Pharyngeal slits
💡Muscular post anal tail
💡Subphylum
💡Molecular evidence
💡Filter feeding
💡Atrium
Highlights
Introduction to two groups of organisms, sea squirts and lancelets, as proto-chordates or invertebrate chordates.
Importance of proto-chordates in understanding the early evolution of the phylum Chordata.
Inclusion of sea squirts and lancelets in the invertebrates 3 laboratory exercise.
All chordates share four derived characters: notochord, dorsal hollow nerve cord, pharyngeal slits, and a muscular post-anal tail.
Proto-chordates are classified into two subphyla: Cephalochordata and Urochordata.
Lancelets are placed in the subphylum Cephalochordata.
Tunicates or sea squirts are placed into the subphylum Urochordata.
Molecular evidence has prompted a revision of the traditional classification system of chordates.
Lancelets' common name 'lens' is derived from their shape, and they lack eyes but have simple eye spots for light detection.
Lancelets have a segmented body with myomeres, similar to fish and ancestors of annelids and arthropods.
Lancelets lack paired appendages and are filter feeders burrowed into the sandy bottom of shallow waters.
Internal anatomy of lancelets includes a notochord, nerve cord, digestive system, and absence of certain structures like eyes and paired appendages.
Lancelets' digestive system includes a buccal cavity, pharynx, atrium, esophagus, stomach, intestine, and anus.
The lancelet's wheel organ and cilia play a crucial role in drawing in food and water.
The hepatic cecum in lancelets is homologous with the liver in vertebrates and aids in digestion and absorption.
Adult tunicates are sessile and filter feed, lacking the derived characters of chordates seen in their larval form.
Tunicates' pharynx resembles a net, capturing planktonic creatures for filter feeding.
Adult tunicates have an incurrent and excurrent siphon for water flow and food processing.
Laboratory resources include models and slides to study the internal structures and anatomy of lancelets and tunicates.
Transcripts
hello
in this recording we will look at two
groups of organisms that together are
informally referred to as proto
chordates or invertebrate chordates they
are sea squirts or tunicates and
Lancelot's neither group is common or
well known but they are important in
biology because they help us to
understand the early evolution of the
phylum to which we belong these
organisms are included in the
invertebrates 3 laboratory exercise sea
squirts and Lancelot's are included in
the phylum Chordata the same phylum to
which vertebrates are assigned there are
4 derived characters shared by all
chordates at least sometime and their
life cycle a notochord a dorsal hollow
nerve cord pharyngeal slits and a
muscular post anal tail proto chordates
are placed into two sub phyla this is a
new classification category for us
subphylum falls between the phylum and
class levels in the taxonomic hierarchy
Lancelot's are placed in the phylum
cephalic or data tunicates or sea
squirts are put into the subphylum
eurocorp data in the past all other core
dates were included in a sub pileup
called vertebrate ax but molecular
evidence and its are to make all groups
monophyletic have prompted taxonomist to
revise this system as we will discuss in
lecture let's look at resources for
learning about Lancelot's your textbook
has useful information about these
organisms in the chapter on vertebrates
here is the model of a lancelet that is
available to you in the laboratory the
anterior is on the left observe the
overall shape it is this that gave rise
to the common name lentil it and to the
no longer use genus name amphioxus which
means pointed at both ends the anterior
most structure is the rostrum the
tentacle like structures Arbuckle seri
which surround the mouth the light color
grid on top is the dorsal fin and the
fins on the posterior end are called
caudal fins notice the v-shaped
structures on most of the body surface
these are my omere's also known as
segmental muscles they are visible
through the thin epidermis of the land
slit the arrangement is similar to that
seen in fish and provides evidence that
the earliest Corps date had a segmented
body as indeed to the ancestors of
annelids and arthropods in addition to
the parts that are present notice the
structures that are absent as compared
to fish there are no eyes although there
are simple eye spots on the nerve cord
for light detection only there are no
paired appendages such as pectoral fins
or pelvic fins
although lance looks can swim weakly
they are filter feeders that spend most
of their time burrowed into the sandy
bottom of shallow coastal waters let's
remove the side of the landslip model
and observe the internal organs the dark
pink layer just beneath the dorsal fin
is muscle part of the my omere's ventral
to the muscle the light pink structure
is the nerve cord all chordates have the
nerve cord in the dorsal side of the
body notice an eye spot near the front
of the nerve cord as mentioned earlier
this
can detect the presence or absence of
light but cannot form images the green
line represents the notochord the
structure that gives us vitamins name
the notochord is a semi flexible rod
that stiffens the body and provides
resistance to the effects of muscular
contraction making coordinated movement
possible in vertebrates this structure
is replaced by the vertebral column
during early development the mostly
yellow structures constitute the
digestive system let's identify the main
parts the orange colored space is the
buccal cavity the narrow region is the
velum we will examine this more closely
in a moment
this large region is the pharynx with
its Gill bars and gill slits as we will
see shortly a water chamber known as the
atrium surrounds the pharynx a short
narrow passageway the esophagus leads
into the stomach this narrow tube is the
intestine which ends at the anus now
let's take a closer look at the anterior
portion of the digestive system note
again the buccal Seri the buccal cavity
the velum and the pharynx the lanceolate
does not have jaws nor does it have a
muscular pharynx that could expand to
suck in food as we saw in earthworms
instead the organism must depend on
cilia to draw water and food into the
digestive system at the front of the
velum are a number of grooves
collectively known as the wheel organ
these structures as well as the inner
surface of the pharynx are covered in
cilia in addition a layer of mucus is
secreted on these surfaces beating cilia
set up a current that draws in
microscopic organisms floating in the
water
many of these organisms stick to the
mucus layer and are transported along
with the mucus into the stomach the
water passes through the gill slits
these openings here into the atrium the
atrium is a chamber that surrounds the
pharynx on three sides you can observe a
portion of it here beneath the pharynx
here is a view of the rear portion of
the digestive system on the left you can
see the pharynx with its gill slits
water passing from the pharynx moves
posteriorly through the atrium
ultimately exiting through the atria
poor food trapped in the mucus within
the pharynx is carried into the stomach
here the process of digestion and
absorption begins aided by enzymes that
are secreted by the hepatic cecum which
I will show you shortly
digestion and absorption continue as the
food moves through the intestine and any
undigestible material passes out through
the anus here is a view of the middle of
the lanceolate in which a portion of the
pharynx has been removed this allows us
to see the hepatic cecum which is
attached to the stomach this blind ended
pouch secretes enzymes and takes part in
the absorption of nutrients evidence
indicates that this structure is
homologous with the liver in vertebrates
the blue structures visible behind the
hepatic cecum are gonads they may be
either testes or ovaries now we will
look at a cross-section of a Lancelot
this slide is available in the
laboratory this circular structure is
the notochord just dorsal to the
notochord is the nerve cord and these
large stringy areas of tissue are my
Amir's that is they are the muscle
this circular structure is the pharynx
the middle is the lumen of the pharynx
these dark structures are the gill bars
and the lighter regions between the Gill
bars are the gill slits the white space
surrounding the pharynx is the atrium to
make sense of the other structures in
the cross-section let's look briefly
again at the model this dotted line
indicates the approximate location of
the cross section of land slit that you
are seeing note that a portion of the
pharynx is removed revealing the hepatic
cecum and the gonads in the model
looking back at the cross-section this
structure to the left of the pharynx is
the hepatic cecum the large dark
staining structures here in the lower
left and the lower right are the gonads
in this case ovaries now let's take a
brief look at the other subphylum
included in the proto chordates the
tunicates which are in Euro core data
once again your textbook has useful
information that will help you in your
laboratory studies looking at the
diagram of an adult tourniquet it may be
unclear why these organisms are included
in the phylum Chordata since they appear
to lack three of the four derived
characters of the phylum however the
diagram of the larval form of the
organism reveals all four of the
expected characters the adults being
sessile which means permanently attached
to a substrate lose unneeded structures
including these three derived characters
here is a hole mount slide of a tunic
--it that is available in the laboratory
it has been treated chemically to make
internal structures visible adult
tunicates are permanently attached to a
substrate and gather food by filter
feeding water is drawn into the organism
through the incurrent siphon by the
beating of cilia water flows into the
pharynx which as you can see occupies
most of the body of the tunic --it there
are no obvious gill slits rather the
pharynx resembles a densely woven net as
water passes through this net planktonic
creatures adhere to the mucus coating
the water enters the atrium which
surrounds the pharynx on all side but is
most easily visible here on the right
water exits through the X currant siphon
the food Laden mucus
which is within the pharynx is carried
down to the digestive system for
processing as always any undigestible
material is expelled through the anus
this as you can see is located near the
ex current siphon for obvious reasons
that concludes our look at the specimens
and models of Lancelot's and tunicates
that are available in the laboratory I
hope this helps you to prepare for the
upcoming laboratory practical
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