Protochordates

Summary

This 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

00:00

🐚 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.

05:01

🌊 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.

10:03

🌊 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.

Keywords

💡Proto chordates

Proto chordates, also known as invertebrate chordates, are a group of organisms that are important in understanding the early evolution of the phylum Chordata, which includes vertebrates. They are characterized by certain features that are present in the phylum, but they lack the full complexity of vertebrates. In the script, they are discussed as organisms that help us understand the evolutionary path that led to the development of more complex chordates.

💡Tunicates

Tunicates, also known as sea squirts, are a type of marine invertebrate that belongs to the phylum Chordata. They are significant in the study of chordate evolution. Adult tunicates are sessile and filter feed, but their larval form exhibits key chordate characteristics. The script mentions tunicates as a subphylum within proto chordates, highlighting their transitional role in the evolution from invertebrates to vertebrates.

💡Lancelets

Lancelets are another group of proto chordates that are closely related to vertebrates. They possess features that are fundamental to the phylum Chordata, such as a notochord, a dorsal nerve cord, and pharyngeal slits. The script describes a model of a lancelet, noting its body structure and the presence of these key features, which are crucial for understanding the early stages of chordate evolution.

💡Notochord

The notochord is a semi-flexible rod that provides support to the bodies of chordates. It is a key characteristic of the phylum and is present in all chordates at some stage of their life cycle. In the script, the notochord is mentioned as a defining feature of lancelets, serving as a precursor to the vertebral column in vertebrates.

💡Dorsal hollow nerve cord

A dorsal hollow nerve cord is a tubular structure that runs along the back of all chordates, serving as the central nervous system. It is one of the defining characteristics of the phylum Chordata. In the script, the nerve cord of lancelets is described as being located dorsally, which is a key feature that distinguishes chordates from other animals.

💡Pharyngeal slits

Pharyngeal slits are openings in the pharynx that are found in all chordates. They are used for filter feeding in some species and for respiration in others. The script explains that lancelets have gill slits, which are a type of pharyngeal opening, and these are important for their feeding mechanism.

💡Muscular post anal tail

A muscular post-anal tail is a feature of chordates that extends beyond the anus and is used for locomotion. The script mentions that proto chordates have this tail, which is a characteristic that is present in the life cycle of all chordates and is indicative of their evolutionary significance.

💡Subphylum

A subphylum is a taxonomic rank that falls between the phylum and class levels in the biological classification system. In the script, the lancelets are placed in the subphylum Cephalochordata, and tunicates in the subphylum Urochordata, indicating a new classification that separates these groups from the traditional vertebrates.

💡Molecular evidence

Molecular evidence refers to the data derived from the study of DNA, RNA, and proteins, which can provide insights into the evolutionary relationships between organisms. The script mentions that molecular evidence has prompted taxonomists to revise the classification system of chordates, indicating the importance of this evidence in modern biological research.

💡Filter feeding

Filter feeding is a method of feeding where an organism strains small particles from water using specialized structures. Both tunicates and lancelets are filter feeders, as described in the script. This feeding mechanism is an adaptation that allows these organisms to extract nutrients from the water in which they live.

💡Atrium

The atrium is a water chamber that surrounds the pharynx in some chordates, including lancelets. It plays a role in the respiratory process by allowing water to flow over the gills for gas exchange. In the script, the atrium is described as a structure surrounding the pharynx of lancelets, where water exits after passing through the gill slits.

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