Invertebrate Anatomy OnLine
Copyright 2005 by
is one of many exercises available from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises, a glossary, and chapters on supplies and laboratory techniques are
also available at this site. Terminology
and phylogeny used in these exercises correspond to usage in the Invertebrate
Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated
figure callouts refer to figures in the textbook. Callouts
that are not hyphenated refer to figures embedded in the exercise. The glossary
includes terms from this textbook as well as the laboratory exercises.
Urochordata sP, Ascidiacea C,
is characterized by a suite of apomorphies including a dorsal hollow nerve cord,
notochord, pharyngeal gill slits, and a post anal tail (Fig 29-1). The
ancestor was a fishlike deuterostome that swam using alternating contractions of
right and left longitudinal axial muscles to create undulations of the body. The
flexible, incompressible notochord prevented these contractions from compressing
the body while allowing lateral deflection. The
chordate central nervous system is a hollow, median, longitudinal nerve cord
formed in the embryo by an invagination of surface ectoderm whose original
function was probably sensory reception. Paired pharyngeal gill slits connect
the lumen of the pharynx with the exterior and originally functioned in
suspension feeding with respiration being added later. A
muscular tail posterior to the anus is, although commonplace in chordates, an
unusual feature not found in other taxa. It
is an extension of the axial musculature and is the chief locomotory organ. An
additional apomorphy is the endostyle, a region of pharyngeal endoderm, that
secretes iodated compounds, either mucus or hormones.
Tunicata (= Urochordata) sP
are highly derived and less like the ancestral chordates than are
cephalochordates or vertebrates. At
some time in the life cycle all possess a notochord, dorsal hollow nerve cord,
pharyngeal gill slits, postanal tail, and endostyle but only the gill slits and
endostyle are present in adults. Adult tunicates use the pharyngeal gill slits
for suspension feeding. The larva is much more chordate-like than the adult and
resembles a tadpole or fish, has all the chordate apomorphies, and is known as
the tadpole larva. Metanephridia are absent and coelom is reduced to a
pericardial cavity and gonads. As in cephalochordates the gut is dominated by an
enormous pharynx surrounded by a water-filled atrium but, unlike
cephalochordates, it is U-shaped with the mouth and anus anterior. Tunicates may
be benthic or planktonic and solitary or colonial. All are marine.
is traditionally divided into Ascidiacea (the benthic sea squirts in three taxa;
Aplousobranchia, Phlebobranchia, and Stolidobranchia), Thaliacea (the pelagic
salps, pyrosomes, and doliolids), and Appendicularia (the pelagic larvaceans). Recent
molecular evidence and reevaluation of morphological evidence, however, suggests
that Ascidiacea is paraphyletic and Tunicata should be reorganized into three
different higher taxa (Fig 29-32). In
this reorganization Stolidobranchia would be one higher taxon. Phlebobranchia
plus Thaliacea would be the second taxon. Aplousobranchia
plus Appendicularia is the final tunicate taxon. For now, however, the
traditional classification will be followed.
is usually taken as representative of Tunicata, at least for the purposes of
introductory laboratory exercises. Ascidians, or sea squirts, are sessile filter
feeders that, as adults, bear little resemblance to their chordate relatives. Ascidians
have a living, external, cellular exoskeleton, or tunic, underlain by epidermis. The
tunic resembles connective tissue, except it isoutside the
epidermis, and consists of cells, a secreted extracellular matrix, and ground
of it is a cellulose-like polysaccharide. In
many ascidians blood vessels cross the epidermis to enter the tunic, a feature
found in no other animal.
gut is U-shaped and both openings are anterior, with the anus dorsal to the
gut is dominated by an enormous pharynx whose wall is perforated by numerous
tiny gill slits. The
pharynx is surrounded by a water-filled atrium into which the gill slits open
and which itself opens to the sea. It
is both respiratory organ and filter-feeding device. Water
and food particles enter the pharynx and the water passes through the gill slits
to the atrium and then out the siphon. Food,
entangled in mucus secreted by the endostyle, remains in the gut and passes
posteriorly to be digested.
hemal system includes a heart, vessels, and blood spaces in the connective
tissue. The heart is enclosed in a pericardial cavity derived from the ancestral
pattern of blood flow resembles that of the cephalochordates and early
vertebrates except that the heart reverses direction periodically and the blood
thus flows in both directions through the system. Ascidians
have no structure recognizable as a kidney.
are simultaneous hermaphrodites and the gonoducts open into the atrium. Some
ascidians are solitary and may be relatively large. Others
are colonial with tiny individual zooids in a common tunic.
gonads are in the gut loop and the neural gland is below the cerebral ganglion.
have no postabdomen and may be solitary or colonial. The pharynx wall has raised
longitudinal and transverse blood vessels but is not pleated. The inner surface
of the pharynx bears projecting papillae that support the mucous feeding net. Papillae
on the inner wall of the pharynx help hold the mucous net in place. No
postabdomen is present. The tail of the tadpole is vertical in all except Perophora.
a colonial, tropical sea squirt whose individual zooids reach lengths of about 2
consist of clumps of closely spaced zooids with elongate posterior pedicles
arising from a common stolon. Each
zooid has its own tunic and complete complement of organs but blood vessels in
the stolon maintain communication between zooids. The
tunic is transparent and there is relatively little body wall musculature,
making these animals ideal for study without dissection. Most
of the internal anatomy is visible through the tunic. Variable amounts of orange
pigment occur in the mantle and is clearly visible externally in living
orange color is lost in preserved material. Ecteinascidia retains
and gestates its orange eggs in the oviduct and atrium and releases tadpole
larvae from the atrium. Developmental
stages, including the tadpole, are bright orange.
specimens can be collected on tropical or subtropical coasts or purchased from
biological supply companies located in such areas. Ecteinascidia is
available commercially as stained wholemount slides and this exercise is based
on such slides. Commercially prepared slides each have a single zooid (Fig 1).
At about 2 cm Ecteinascidia zooids
are larger than is usual for compound ascidians making it large enough to see
most of its structures without difficulty. .
living specimens are available, water circulation and feeding should be observed
through the transparent tunic using seawater tinted with non-toxic dyes (food
coloring) or laden with suspended particles (carmine, India ink). For
observation of smaller, less visible structures such as the beating heart and
developmental stages the zooid can be easily removed from the tunic using fine
forceps and iridectomy scissors.
a wholemount of Ecteinascidia using
20X of the dissecting microscope. Ecteinascidia is
a colonial ascidian in which individual zooids, each with a complete complement
of viscera, are connected by a creeping stolon (Fig 29-12B). The
zooid is elongate with a long tapered pedicle which
connects with the horizontal stolon. The
stolon may not be represented on the slide. A blood
vessel extends from the
zooid through the pedicle to the stolon. The zooid on the wholemount has been
separated from the other zooids of its colony. It
is probably distorted due to contraction during the fixation process and because
it is severely depressed by the coverslip.
wide blunt end of the zooid is anterior and
the tapered end is posterior (Fig
1). Specimens on wholemounts are usually mounted so you are viewing either the
right or left side. Be
careful in the instructions that follow to distinguish between the animal’s
right and left and directions on the slide. The ventral aspect
is best recognized by finding a very long bright red (stained) groove, the
endostyle. The endostyle begins near the buccal siphon and extends posteriorly
for the length of the pharynx. The opposite edge, with the atrial siphon and
rectum is dorsal.
Figure 1. A commercially prepared wholemount of a
stained Ecteinascidia zooid
viewed from its left side. Uro98L.gif
proceeding find the above landmarks and determine if you are looking at your
specimen’s right or left side. As
always, directions refer to the animal’s right and left, not necessarily yours. It
is best to conduct most of your study with the left side uppermost, facing you.
You can turn the slide over if necessary but be careful you don’t damage the
coverslip. To be safe, suspend the slide (upside down) from two applicator
sticks, one at each end of the slide, so the coverslip does not touch the stage.
zooid is elongate with the anterior end (thorax) wider than the tapering
posterior pedicle. (Distortion of the tunic may cause the posterior end to be
wider than the anterior.) The transparent tunic is
lightly stained (usually green) and surrounds the zooid. At the anterior end
find the two siphons. The buccal
siphon is near the center of
the anterior end and the atrial
siphon is dorsal to it (Fig
opening of both siphons is flush with the truncate anterior end of the thorax
but may not appear so in wholemount slides. If the specimen is distorted the
siphons may not be at the tip of the body but posterior to it.
buccal siphon is the anterior opening of the digestive system. The
mouth lies just inside the siphon and is encircled by a ring of buccal
tentacles, but these may be difficult to find in wholemounts. The
buccal siphon and mouth open into the pharynx, which is the region of the gill
slits and the site of filtration. The
pharynx is surrounded by a water jacket, the atrium which empties to the
exterior through the atrial siphon. The
atrium receives seawater that exits the pharynx through the gill slits, feces
from the gut, and gametes from the hermaphroditic gonads. In many species,
including Ecteinascidia, it
also functions as a gestation chamber.
enormous, pale pink (stained) pharynx occupies
most of the thorax. It
is, of course, the anterior region of the gut into which the buccal siphon
is adapted for filter feeding and its walls are perforated by abundant small
oval gill slits. In
most specimens about 30 rows of gill slits, with about 60 slits per row, are
are looking through both sides of the pharynx so you will see gill slits on its
right and left sides superimposed on each other in a confusing jumble. Examine
the pharynx with 100X of the compound microscope to see the gill slits more
clearly but do not use high power on these thick slides.
the slide to the dissecting microscope and 20X. The endostyle is
a long, conspicuous, red-staining groove extending the length of the ventral edge
of the pharynx. It
will be a narrow pale stripe flanked on each side by a thick heavily stained
appearance is due to the endostyle being a groove flanked by two thick ridges of
tissue (Fig 29-15C).
The esophagus exits
the pharynx dorsally at the posterior end (Fig 1). It is a short curved tube. When
viewed from the right, the esophagus exits the dorsal posterior corner of the
pharynx. The esophagus extends to the left and joins the stomach,
which is a large chamber on the posterior margin of the pharynx. It extends from
dorsal to ventral across the posterior end of the pharynx. Upon
reaching the ventral edge of the zooid it narrows and curves anteriorly to pass
diagonally to the left of the pharynx as the intestine. The
transition from stomach is the pylorus and here the gut receives a duct from the
digestive ceca. The
ceca are not usually visible in these preparations. The
intestine extends anteriorly then angles obliquely across the left side of the
pharynx and becomes the rectum. The rectum extends
straight anteriorly on the dorsal margin of the pharynx (Fig. 1). The appearance
of the intestine and rectum vary depending on their contents, if any. They
may be yellowish, or dark, or pale pink (if empty). The yellow color is due to
the contents. Its
walls can be seen to stain pink like the other regions of the gut. The rectum
terminates at the anus beside
the atrial siphon at the anterior end.
anus empties into the atrium, which
encloses the right, left, and dorsal sides of the pharynx. It
can be seen along the dorsal margin of the pharynx, between the pharynx and the
in the loop between the esophagus and the intestine, on the left of the animal,
for the gonads. The testis in
a horseshoe shaped, lobed, intensely stained, red circle in this loop. A
distinct dark red sperm duct extends
from one end of the testis to the atrium where it opens near the anus. The
yellow ovary is
a cluster of large eggs located in the center of the horseshoe of the testis. The
oviduct extends from the ovary to the atrium but it does not follow the sperm
it reaches the atrium at a more posterior position and extends over the dorsal
margin of the pharynx to opens into the right side of the atrium (Fig 1). The
oviduct is difficult to see unless it contains embryos being gestated.
heart, cerebral ganglion, neural gland, and digestive ceca are not usually
visible in these wholemounts.
Berrill NJ . 1950.
The Tunicata, with an account of the British Species. Ray Society, 133: 1-354.
Ruppert EE, Fox RS. 1988. Seashore
animals of the Southeast. Univ.
South Carolina Press, Columbia. 429pp.
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
Van Name WG. 1921. Ascidians
of the West Indian Region and the Southeastern United States. Bull. Am. Mus.
Nat. Hist., 44(16): 283-494.
Van Name WG. 1945.
The North and South American ascidians. Bull. Am. Mus. Nat. Hist., 85:1-476, pls
preserved, or wholemounts
Wholemount slides are available from Carolina Biological
( www.carolina.com )
Living specimens from Gulf Specimen Marine Laboratories Inc ( www.gulfspecimen.org ).
Compound and dissecting microscopes