Invertebrate Anatomy OnLine
Copyright 2005 by
This 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.
Chordata P, Urochordata sP, Thaliacea C, Doliolida O, Doliolidae F
Chordata 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
Tunicates 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.
Tunicata 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.
Thaliaceans are free swimming planktonic urochordates. The buccal and atrial siphons are at opposite ends of the body. In most the ciliary feeding current is also responsible for locomotion. The taxon includes pyrosomes, doliolids, and salps.
Thaliaceans with barrel-shaped zooids with 8-9 circumferential muscle bands in the body wall. Complex life cycle includes a solitary sexual generation and colonial asexual generation.
This exercise is based on gonozooids collected from the Gulf Stream off North Carolina in 1973. Preserved material and prepared slides are not commercially available. Doliolumzooids are transparent and it will be beneficial to stain specimens. The specimens used for this study were stained with amido black. Specimens should be mounted on slides and examined with low power of the compound microscope. If available, living specimens should be examined in seawater or isotonic magnesium chloride with the dissecting microscope.
The gonozooid is the solitary, sexually reproducing generation of the doliolid life cycle (Fig 29-28). It develops asexually from a phorozooid and then breaks free and becomes solitary. During its planktonic life it releases gametes. Fertilization produces a zygote which develops into a tadpole larva, also in the plankton. The tadpole matures into a colonial oozooid, or nurse. The oozooid gives rise, through asexual budding, to blastozooids which mature to become the diverse zooids of a colony that includes the original oozooid plus trophozooids and phorozooids. Trophozooids are responsible for feeding the colony, whereas phorozooids eventually leave the colony. They, the phorozooids, produce gonozooids to complete the life cycle.
Examine a wetmount or wholemount of a stained specimen with 40X. Observe the barrel shape characteristic of doliolids (Fig 1, 29-28C). The body is covered on the outside by a thin, elastic tunic. The anterior end is occupied by a large opening, the buccal siphon. The margins of its aperture are scalloped with 10 buccal lobes which bear sensory receptors for monitoring the incoming water and food stream (Fig 29-28A, but not visible in Fig 1). The buccal siphon opens directly into the pharynx, which fills the anterior two thirds of the body. The pharynx wall is perforated by gill slits. The endostyle is a ciliated groove visible as a longitudinal line on the ventral midline of the pharynx.
Posteriorly the pharynx narrows to form a short esophagus which quickly dilates to become the stomach. The intestine leads from the stomach to the anus. Fecal pellets may be visible in the intestine. The anus opens into the large, spacious atrium.
Figure 1. Ventral view of a Doliolum gonozooid (stained with amido black). Uro97L.gif
Lateral cilia on the gill slits generate a feeding current that enters the buccal siphon and passes posteriorly into the pharynx. The long lateral cilia are clearly visible protruding into the gill slits when viewed with 100X. They extend almost half way across the gill slit. From the pharynx water passes to the side through the gill slits and into the surrounding atrium. The endostyle produces a mucous sheet that is moved dorsally to cover the inside wall of the pharynx, including the gill slits (Fig 29-15D). The mucus acts as a filter to remove fine food particles from the water current as it passes through the gill slits. The mucus and entangled food particles are moved posteriorly by cilia into the esophagus and stomach where digestion takes place.
The atrium is a water space surrounding the pharynx and extending posteriorly from the pharynx to the atrial siphon. Being a space, the atrium is not really visible, and you have to accept its presence on faith unless you have the wherewithal to make cross sections. The atrial siphon is the large opening at the posterior end of the body. It is ringed by 12 atrial lobes similar to the buccal lobes at the anterior end.
The body is encircled with eight muscle bands that are clearly visible in stained material. These are the swimming muscles whose contractions force a water jet out the atrial siphon, resulting in the forward motion of the animal. The muscle bands are antagonized by the elastic tunic.
The hermaphroditic gonad is visible as an elongate sac beside the posterior pharynx. It opens into the atrium.
The nervous system, which includes a cerebral ganglion and a single statocyst (on the left side) will not be studied. The neural gland is a ciliated duct arising in the anterior dorsal pharyngeal wall ventral to the ganglion. It will probably not be seen but its function is to replace blood plasma, lost by leakage across the pharynx epithelium, with seawater.
Bullough WS. 1958. Practical invertebrate anatomy, 2 nd ed. Macmillan, London. 483 pp.
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
Slides and coverslips