Hello marine biology students.
In this last video of the week we're going
to be talking about invertebrate chordates.
[Intro Music]
Phylum Hemichordata or the hemichordates 
contains several of the features of chordates
but are lacking just a few.
Shared components include a pharyngeal gill
slit
and a hollow dorsal nerve cord.
The hemichordates are characterized by acorn
worms which are deposit feeders that use a
proboscis for feeding.
There are some invertebrate chordates in our
own phylum, phylum Chordata.
So the chordates include two invertebrate
groups, the tunicates and the lancelets, as
well as all of the vertebrates, which include
the fish, amphibians, reptiles, birds and
mammals.
The chordate characteristics that must be
in all chordates, at least at some stage in
their life, include a hollow dorsal nerve
cord, a notochord, gill slits along the anterior
half of the pharynx, and a post anal tail.
Since humans are vertebrates, we have all
of these characteristics at least at some
stage during our development.
Looking at the invertebrate chordate groups,
the first we'll talk about are the tunicates
in the subphylum Urochordata.
For the tunicates, only their larvae, which
are tadpole larvae, have chordate characteristics.
These are not seen in the attached sack like
adults, except the pharynx with gill slits.
This includes the sea squirts or ascidians
who are sessile filter feeders.
They have an outer covering known as a tunic
which is a thick outer layer for their sack
like body and it is cellulose-based.
Another type of tunicate is a planktonic adult
known as a salp and it retains much of its
larval features even in adult form.
Sea squirts or ascidians, they can be colonial
or solitary, as we see in the photograph.
In the illustration we can see the gill slits
and the pharynx and then to the far right
we see a salp, where the outer gelatinous
house is made out of cellulose and is considered
the tunic whereas the larvacean is the main
animal itself, which is feeding and swimming,
providing the life of this structure.
Our last group of invertebrate chordates are
the lancelets, subphylum Cephalochordata or
the cephalochordates.
They possess all chordate features as an adult.
They have a fish-like body, yet with the absence
of a backbone, which is why they are considered
an invertebrate.
Lancelets are small, usually only up to 3
inches long or so.
There are filter feeders who live in shallow
soft sediments
and they make use of their gills to help in
the filtering of food.
So here we see an adult lancelet or amphioxus.
We see its mouth, pharyngeal gill slits, a
notochord and a hollow dorsal nerve cord,
a complete digestive system, and a post anal
tail.
All of these essential chordate characteristics.
So that was a lot of different animal phyla
that we discussed.
One of the goals of taxonomy and systematics
is to try to arrange organisms in a meaningful
way based on their evolutionary paths and
relatedness to each other.
So here we see one proposed phylogenetic relationship
of several of the animal phyla we talked about
in this week's videos.
This is based on morphology, it's based on
developmental studies, but as we continue
to do genetic studies and molecular biology
studies, you might find that some of these
organizational patterns aren't necessarily
set and that we would expect to see variations
in the organization or at least the categorization
of their relatedness of these animal phyla.
As with previous chapters, there's an incredible
table highlighting the different animal groups
we've talked about in these videos, talking
about their similar features and how they
are different from each other.
This is Table 7.1 in the textbook and I highly
encourage you to look at that table in the
textbook because it's hard to see all the
detail of this table in this slide.
And that concludes our discussion of invertebrate
chordates.
Now, before our videos next week, I would
like you to consider what organisms would
you likely go to an aquarium to see.
We'll probably be talking about them next
week.
See you then.
