Practical Course in Zoology: Study Guide - T. A. Dauda 2014

Multicellular Animals
Chordates
Subphylum Acrania – Class Leptocardii

General characteristics

Acrania (Amphioxus and allies) are exclusively marine, primitive Chordates that retain all the main chordate features throughout their lives: a notochord, a nerve cord, and gill slits in the anterior part of the gut. A distinct Brain is absent.

The subphylum is represented by a single Class — Cephalochordata. It includes about 30 species of lancelets inhabiting the waters of the Atlantic, Indian, and Pacific Oceans.

Lancelet (Branchiostoma lanceolatum).

Materials and equipment. The lancelet is a small (up to 10 cm) animal found in shallow coastal waters, where it burrows into sandy seabeds. Only rarely does a lancelet swim freely. In our region, it occurs in the Black Sea. The entire body of the lancelet is transparent, allowing most of its Internal Organs to be observed in a whole mount.

Required equipment and materials: a compound Microscope; a dissecting loupe; a hand lens; a watch Glass; a large lancelet specimen preserved in 70% alcohol; permanent microscope slides (a stained whole mount of the lancelet, a cross-section through the pharyngeal region of the gut, and a cross-section through the digestive region of the gut); and a diagram of the lancelet Circulatory system.

Fig. 113 Internal Anatomy of the lancelet (Branchiostoma lanceolatum):

1 — oral hood surrounded by cirri; 2 — caudal fin; 3 — dorsal fin; 4 — Muscle segments (myomeres); 5 — nerve cord; 6 — notochord; 7 — gill slits; 8 — gills; 9 — intestine; 10 — hepatic caecum; 11 — Gonads; 12 — atriopore/atrial cavity.

Assignment. Examine and draw the General Anatomy of the lancelet. Examine and draw cross-sections of the lancelet through the pharyngeal region and the digestive region of the gut. Study the circulatory system using the provided diagram. Identify the Characteristic Features of the phylum Chordata exhibited by the lancelet, and note the primitive traits in its bodily Organization.

Draw the overall outline and Internal Structure of the lancelet; cross-sections of the body in the pharyngeal and intestinal regions; and the diagram of the lancelet circulatory system.

Study of the lancelet anatomy (using preserved specimens and a stained whole mount).

Take a large lancelet specimen preserved in 70% alcohol, place it in a watch glass, and examine it with a hand lens while turning it gently with forceps. Note that the body is relatively small (5–8 cm), laterally compressed, and elongated; it is more tapered at the posterior end, while the anterior end appears obliquely truncated. The ventral side is broader, whereas the dorsal side is narrow. The outer integument is transparent and unpigmented. The body color is whitish (pinkish-white in living lancelets).

Fig. 114 Anterior end of the lancelet (Branchiostoma lanceolatum):

1 — oral cirri; 2 — notochord; 3 — olfactory pit; 4 — frontal eye spot; 5 — nerve cord; 6 — Hesse ocelli; 7 — finger-like processes of the velum.

Continue The Study of the external anatomy of the lancelet using a stained whole mount under a dissecting loupe or low magnification of a microscope (Fig. 113).

Observe that a fin fold—a low dorsal fin—runs along the dorsal side. It is transparent and supported by numerous well-defined, rod-like fin rays. Moving the slide from the broader anterior end toward the posterior, notice that the dorsal fin continues around the tail end to form the caudal fin, which is shaped like a spearhead or lancet (hence the animal's common name, lancelet), and extends onto the ventral side as a short ventral fin. Locate the oral cirri at the anterior end of the body near the ventral side (Fig. 114).

From the oral cirri along the sides of the ventral region, two prominent metapleural folds extend all the way back to the ventral fin. At the point where they meet the ventral fin lies the opening of the atrial (or peribranchial) cavity, known as the atriopore, or gill pore. The atriopore is not visible when the lancelet is lying on its side, but its Location can be determined.

The entire body of the lancelet is so transparent that most of its internal organs can be seen by examining the specimen in transmitted light. The musculature of the lancelet is clearly visible—it is metameric (an invertebrate trait), consisting of muscle segments called myomeres. The myomeres show through the Skin, revealing that they are separated from one another by thin Connective Tissue partitions known as myosepta. The myomeres are shaped like triangles with their apexes pointing toward the anterior end of the body. A layer of transverse Muscles lies across the ventral side of the lancelet.

Draw the outlines of the lancelet body, marking the fins, metapleural folds, atriopore, and the Mouth opening surrounded by cirri. As you study the internal structure, add the details into this outline.

It is best to begin studying the internal anatomy of the lancelet from the anterior end. Locate the mouth opening surrounded by cirri, situated on the ventral side of the anterior (HEAD) end.

Examine the circular membrane (velum) with its finger-like projections, which separates the Oral Cavity from the Pharynx. On the DORSAL SIDE OF the body, locate the notochord, which extends far forward into the head region. The notochord, or dorsal string, serves as the axial Skeleton of the lancelet. It appears as a pale, vertically striated rod running along the dorsal side from the anterior to the posterior end of the body.

Note that the nerve cord lies directly above the notochord; it contains a fluid-filled cavity known as the neurocoel. The nerve cord of the lancelet (the Central Nervous system) is not differentiated into a BRAIN AND SPINAL cord. The lancelet lacks a Skull (hence the subphylum name, Acrania). Under low microscope magnification, locate the slightly expanded anterior tip of the nerve cord, which features a pigment spot known as the frontal eye. A series of black dots resembling a dashed line—Hesse ocelli (photoreceptor organs)—are distributed along the nerve cord. These are best observed by adjusting the fine focus knob and changing the illumination.

In the anterior half of the body, beneath the notochord, lies the atrial or peribranchial cavity, which opens to the exterior via the atriopore. Occupying a considerable space within the lancelet's body, the atrial cavity has compressed the true coelom (secondary body cavity), of which only scattered remnants persist. In the anterior body region, coelomic remnants surround the gonads and also form supra-pharyngeal and sub-intestinal canals. In the posterior part of the body, the coelomic cavity surrounds the gut (all of these structures can be clearly observed by examining histological sections).

Digestive and respiratory systems. Moving the specimen from the anterior end of the body to the posterior, examine the amphioxus's interconnected digestive and respiratory systems. Note that the walls of the pharynx are pierced by numerous (up to 150 pairs) obliquely positioned gill slits. The pharynx extends for approximately half the length of the body. Water currents are driven by tentacles into the oral hood and the mouth cavity, which is separated from the pharynx by a circular membrane with processes known as the velum. As the velum expands, water enters the pharynx through the mouth opening, passes through the gill slits into the atrial cavity, and exits outside via the atriopore. Gas exchange takes place as water flows through the gill slits, oxidizing the venous Blood within the vessels located in the gill partitions. Narrowing posteriorly, the pharynx transitions into the intestine.

Fig. 115. Amphioxus nephridia:

A — entire tubule with solenocytes; B — part of the excretory canal wall with attached solenocytes; 1 — upper part of the gill slit; 2 — opening of the nephridial tubule opening into the atrial cavity.

Locate the blind hepatic cecum in the lower part of the middle intestine (adjust the microscope fine focus while altering the illumination). The posterior intestine is a tube (frequently filled with dark contents in the preparation) that terminates in the anus on the left side of the posterior part of the body

of the amphioxus. Food (small marine organisms and food particles) carried into the pharynx by the water current does not escape through the gill slits. The floor of the pharynx features a subpharyngeal groove, or endostyle. Above it, along the upper part of the pharynx, runs the epipharyngeal (suprabranchial) groove. Peripharyngeal grooves connect the endostyle and the epipharyngeal groove anteriorly. Food particles entering the pharynx become coated with mucus and are swept to the floor of the pharynx. Driven by the beating of the cilia in the endostyle, these food clumps move forward along it, then ascend via the Ciliary movement along the right and left Branches of the peripharyngeal ring, and, moving backward along the epipharyngeal groove toward the caudal region, enter the midgut.

Fig. 116. Diagram of the CIRCULATORY SYSTEM OF amphioxus:

1 — ventral aorta; 2 — expansions at the bases of the afferent branchial Arteries; 3 — branchial arteries; 4 — roots of the dorsal aorta; 5 — carotid arteries; 6 — dorsal aorta; 7 — caudal vein; 8 — subintestinal vein; 9 — hepatic portal system; 10 — epibranchial (peribranchial) vein; 11 — anterior cardinal vein; 12 — posterior cardinal vein; 13 — Cuvierian duct.

The excretory system of amphioxus consists of paired nephridia with solenocytes (Fig. 115). Nephridia numbering

90 pairs are located above the pharynx, opening at one end into the coelom (in the supra-pharyngeal canals) and at the other into the atrial cavity. Nephridia are not visible in standard educational preparations.

Reproductive System. Amphioxuses are dioecious. Locate the reproductive glands — gonads, numbering 25–26 pairs, situated along the sides of the body as oval structures. Gonads can only be found by examining preparations of sexually mature amphioxuses; immature specimens lack gonads. In males, the gonads have a fine-grained interior, whereas in females they are coarse-grained. The gonads lack efferent ducts. Germ Cells pass through ruptures in the walls of the gonads and the body wall into the atrial cavity, from which they are expelled outside with water through the atriopore. Fertilization is external.

Examine the circulatory system of amphioxus using the diagram (Fig. 116). The circulatory system is closed, with a single circulatory loop and no Heart. The blood is colorless.

The ventral aorta, located beneath the pharynx, performs the function of The Heart. Venous blood collected from all over the body into the ventral aorta is forced by contractions of its walls into the branchial arteries situated within the gill partitions. Gas exchange occurs within them.

Oxygenated blood flows via efferent branchial arteries into the paired suprabranchial vessels — the roots of the aorta. Paired carotid arteries branch off from them toward the head region. The carotid arteries and the roots of the aorta are located in the anterior half of the amphioxus's body. In the posterior half, the roots of the aorta merge to form a single unpaired dorsal aorta. Arteries branch off from the dorsal aorta, which lies beneath the notochord and extends to the caudal region.

Blood carried through arteries and capillaries—which form a network in various Organs of the body—supplies oxygen to the cells, while Veins transport carbon dioxide and Metabolic waste products away. Capillaries empty into larger veins. Venous blood from the anterior part of the body collects in the paired anterior cardinal veins, where it flows from the anterior end of the body toward the posterior. Veins from the posterior end of the body form the paired posterior cardinal veins, in which blood moves toward the anterior end. Slightly posterior to the pharynx, the anterior and posterior cardinal veins on the right side of the body meet and fuse to form the right Cuvierian duct. The anterior and posterior cardinal VEINS OF THE left side form the left Cuvierian duct. Both Cuvierian ducts empty into the sinus venosus and subsequently into the ventral aorta. In the posterior part of the body, aside from the paired posterior cardinal veins, there is an unpaired subintestinal vein. It forms a capillary network within the hepatic cecum, known as the hepatic portal system. Capillaries of the hepatic cecum converge to form the hepatic vein, which empties into the ventral aorta.

Study of transverse sections of amphioxus. When examining sections for better orientation, it is necessary to compare the observed view with the whole-mount drawing of the amphioxus in your workbook.

Examine the section in the pharyngeal region under high microscope magnification. Pay attention to The structure of the amphioxus skin. The body of amphioxus is covered by a single-layered epithelium (similar to invertebrate animals). The epithelium rests on a thin underlying basement membrane and is covered on the outside by a cuticle. Beneath the epithelium lies a layer of gelatinous tissue known as the cutis. Together, the epithelium and cutis constitute the skin of the amphioxus.

Fig. 117. Transverse section of the amphioxus body:

A — in the pharyngeal region; B — in the intestinal region; 1 — notochord; 2 — Spinal Cord; 3 — dorsal fin; 4 — myomere; 5 — coelom; 6 — pharynx; 7 — gill slit; 8 — interbranchial septum; 9 — endostyle; 10 — peribranchial (atrial) cavity; 11 — Liver; 12 — intestine; 13 — nephridium; 14 — dorsal aorta; 15 — subintestinal vein; 16 — gonads.

Switch to low microscope magnification to locate the dorsal fin, and on the ventral side, find the metapleural folds and the unsegmented transverse muscle located beneath the skin. On the dorsal side, examine the myomeres separated by myosepta (Fig. 117). Located between the myomeres as a large oval is the notochord. Above the notochord, a section of the nerve cord with the neurocoel is visible.

Under high magnification, locate the narrow cleft extending upward from the neurocoel, along with Hesse's ocelli surrounding it. Switch back to low magnification. Beneath the notochord, the pharynx is visible, consisting of rods (gill partitions) separated by gill slits. Examine the epipharyngeal groove on the dorsal side of the pharynx, and flanking it, the coelomic regions (supra-pharyngeal canals) containing the nephridia (which are not visible). On the ventral side of the pharynx, the endostyle is clearly distinguishable, lined with glandular and ciliated cells. In some preparations, the hepatic cecum is visible lateral to the pharynx. On the sides of the pharynx, reproductive glands (gonads) can be observed within the coelomic regions bordering the atrial cavity that surrounds the pharynx.

Determine the sex of the amphioxus based on the Contents of the gonads. Coarse-grained contents represent female ova (egg cells). In males, the gonads are filled with numerous small germ cells.

Draw a transverse section of the amphioxus in the pharyngeal region.

Take a section of amphioxus in the intestinal region (from the posterior part of the body). Follow the same examination Procedure as for the previous section.

Examine and sketch the dorsal fin, myomeres, nerve cord, notochord, intestinal section surrounded by the coelomic cavity, and the posterior part of the atrial cavity.