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PROTOCHORDATES
Introduction to the Protochordates
The protochordates is a term used for the non-craniate chordates, the invertebrate chordates. It
includes two subphyla, the Urochordata and Cephalochordata, and they give us the opportunity
to see the evolution of the phylum from a sessile filter feeder to the swimming fish-like animals
that mark the start of the craniate lineage. As you examine the specimens from these two
subphyla watch for the chordate characteristics: post anal segmentation, notochord, dorsal
hollow nerve chord, the endostyle and finally, the filtering pharyngeal basket.
Urochordata, Ecteinascidia - an ascidian example
The urochordates include a group of marine organisms that spend most of their adult lives as
sessile filter feeders and only a short time as swimming tadpole-like larva. It’s the larval stage
that contains the full set of chordate characters.
Adult anatomy
Ascidian adults are sessile and are found attached to just about any surface in the marine
environment. Most are colonial, and young bud from the parent to create colonies of organisms
with colony members sharing things like excurrent siphons and the tunic, or body covering, that
gives these animals their common name of tunicates. The tunic is composed of tunicin, a
mixture of protein and cellulose-like glucose polymers.
Prepared slides of whole specimens are convenient for identifying the external and internal
features of a tunicate. Be careful with your observations, the relationship of the various internal
structures to each other is distorted when the specimens are flattened onto the microscope slide.
If preserved specimens are available be sure to look at these as well.
The outer body wall, the tunic, is secreted by the underlying epidermis, and preserved
specimens may have a considerable amount of dirt and debris caught in its surface. The tunic is
given this unique name because it isn’t a non-living cuticle. The tunic may contain wandering
amoebocytes and extensions of the underlying epidermis. Ecteinascidia is a colonial tunicate
and the stolon, or stalk is visible at the base of the tunic.
There are two large openings in the tunic and each represents one end of the U-shaped body.
The anterior branchial siphon carries water into the animal and the dorsal atrial siphon carries
it out. The visceral mass at the bottom of the tunic is the posterior side of the animal. The
branchial siphon has a ring of tentacles that surround the mouth and they prevent large objects
from entering and are chemosensory as well. If you look closely at the body wall of both siphons
you’ll see the musculature that opens, closes and changes the diameter of the siphon opening.
Longitudinal muscles are also involved in the closing and opening of the siphons.
Below the incurrent siphon is the large branchial basket. Food laden water is pulled into the
basket and trapped in a sheet of mucus produced by the endostyle, which appears as a dark
groove running the length of the basket. Water that enters the basket, passes through the mucus
bag and the openings in the wall of the basket to the atrium and the excurrent siphon. Carefully
focus on the basket and you’ll see the openings in the wall, the stigmata. They are lined with
cilia and create the water current.
The sheet of mucous produced by the endostyle spreads across the inner surface of the basket
until it reaches the dorsal lamina on the opposite side. Here the mucous bag is rolled up and cilia
pass it down and into the esophagus. The digestive tract is S-shaped and the stomach is found
at the posterior end of the tunicate and the intestine winds its way up and towards the atrial
siphon. You may be able to see the digestive gland on the surface and if you do it’s a convenient
way to tell the difference between the stomach and the start of the intestine.
The other prominent internal organs are the single ovary and testes, located in the fold of the
stomach and intestine. The testes wraps around the ovary and the genital duct leads to a sperm
duct, also located in the atrial siphon.
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Digital Zoology LabManual © Houseman
The heart lies inside a muscular pericardial cavity below the stomach and two blood vessels
supply the viscera, the visceral vessel, or the branchial basket, the branchial vessel. The tunicate
heart is unusual in how it beats, sometimes pumping blood in one direction, and then reversing
the beat to pump it in the other. Depending which type of beat is occurring and when, these
vessels alternate between being arteries and viens!
Larval anatomy
The tadpole larva of the ascidian tunicates is the stage in the life cycle where you will be able
to see the chordate characteristics. In its simplest interpretation this is a swimming branchial
basket. Because you’ll be trying to identify internal structures through the intact body wall of a
whole mount you might want to use a few slides when you make your observations and move
the focal plane through the specimen to see the internal detail.
It’s a tail because it is a segmented extension of the body posterior to the anus, which is located
just inside the atrial siphon. Post anal segmentation is a chordate character. The tail is covered
in a transparent tunic that forms a dorsal and ventral fin and as you look at the prepared slide
you are probably seeing a lateral view of the animal. If that is the case take a quick look at the
anterior end and locate the statocyst and eye. They are on the dorsal side and from this you’ll
know which the dorsal fin is. The notochord is large and uniform in its appearance and above it
is the nerve cord that runs the length of the tail. All of this is surrounded with the muscle cells
that move the tail.
At the most anterior end of the body are the adhesive papilla that the larval stage uses to anchor
to the substrate when it makes its transformation into the adult. The other prominent structures
are the dark statocyst and behind it the pigmented cells of the eye. Both of these sensory
structures are surrounded by the clear sensory vesicle. The opening to the branchial chamber is
just in front of the statocyst and atrial siphon opens behind. It may be hard to see the openings
to the two siphons when they are obscured by the distortion of the body flattened on the
microscope slide. Carefully move your focal plane through the specimen and you should be able
to see the branchial basket and the thickened endostyle that lies underneath it.
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Digital Zoology Lab Manual © Houseman