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

Multicellular Animals
Arthropods
Subphylum Branchiopoda/Crustacea - Class Malacostraca

Unlike other crustaceans, the vast majority of malacostracans possess a fixed number of segments (4 HEAD, 8 thoracic, and 6 abdominal segments). The abdomen bears 6 pairs of appendages. In adults, the excretory Organs are typically antennal glands. A zoea larva is characteristic of their development.

The subclass Malacostraca (comprising over 14,000 species) includes 14 orders, of which only one—Decapoda—will be discussed below.

Order Decapods (Decapoda)

This order unites large and, in many respects, the most highly evolved crustaceans. They are very widely distributed, occurring at all depths in seas and oceans. The fauna of decapods is particularly diverse in shallow tropical waters. Freshwater forms include crayfish, certain crabs, and shrimp. Some crabs and hermit crabs have transitioned to a terrestrial lifestyle.

Female decapods attach their eggs to their abdominal limbs and brood them until the young hatch. Direct development is typical of freshwater and certain deep-sea marine species. Others feature a larval stage known as a zoea.

Many decapod crustaceans are invariably valuable commercial species. Crayfish, crabs, lobsters, spiny lobsters, and shrimp represent valuable food products.

To study Morphology and internal anatomy, it is recommended to use the noble crayfish (Astacus astacus). It should be noted that various manuals may use different Latin names for this species: Astacus fluviatilis, Cancer astacus, Potamobius astacus, etc.

Noble crayfish (Astacus astacus).

The crayfish inhabits rivers, oxbow lakes, and lakes. Most commonly, crayfish stay in burrows dug into the banks underwater. They act as natural scavengers of Water bodies.

Materials and equipment. External anatomy is studied using live specimens or specimens preserved in alcohol or formalin (thoroughly rinsed in running water beforehand). To examine the Skeleton, crayfish are boiled in a 15-20% caustic alkali solution. Internal anatomy is studied by dissecting freshly chloroformed specimens.

The work requires: a dissecting Microscope; a hand lens; watch glasses; beakers with water; pipettes; dissecting tools (tray, scissors, scalpel, dissecting needles, pins, forceps); a sheet of paper or cardboard; crayfish (fixed or live); and a wet mount of a dissected crayfish.

Assignment. Examine the External Structure of the crayfish from the dorsal and ventral sides. Dissect the crayfish, observe the general arrangement of the organs, and prepare and examine the circulatory, reproductive, digestive, excretory, nervous, sensory, and respiratory systems. Sketch the dissected crayfish and its Organ Systems. Dissect the appendages of the crayfish, examine them, and draw them.

Study of external anatomy. Place the alcohol- or formalin-preserved crayfish in dissecting trays, dorsal side up, and examine their external structure. Note that the body of the crayfish is covered with a tough cuticle. It consists of a nitrogenous organic substance, Chitin, and is impregnated with calcium salts. The chitinous shell protects the animal from adverse environmental influences while serving as an external skeleton for Muscle attachment. Examine the body segments—they vary in shape and function, exhibiting heteronomous segmentation. The body regions are the cephalothorax and the abdomen (see Fig. 82).

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Fig. 82 Noble crayfish, dorsal view, and its appendages:

1 — antennule; 2 — spine; 3 — antenna; 4 — claw; 5 — eyes; 6 — carapace; 7 — branchiostegite (gill cover); 8 — cephalothorax; 9 — branchiocardiac grooves; 10 — abdomen.

The cephalothorax is formed by the complete fusion of the head and thoracic segments. It is covered by a common, highly durable chitinous carapace, which is fused to the thoracic segments dorsally and hangs freely on the sides, forming branchial covers (branchiostegites) beneath which lie the gill cavities.

Examine the carapace: it features a wedge-shaped process in front called the rostrum. Two stalked, movable eyes can retract beneath its base. Three grooves are visible on the upper surface of the carapace: a transverse cervical (neck) groove, which separates the head region from the thoracic region, and two longitudinal branchiocardiac grooves. These two grooves are so named because The Heart lies beneath the carapace between them, while the gill cavities are located laterally.

Examine the abdomen, which consists of six movably articulated segments and a terminal plate—the telson—which, together with a pair of greatly flattened appendages on the 6th segment, forms the tail fan. Note that the chitinous cuticle at the junctions between the abdominal segments is very thin and flexible. This ensures the mobility of the abdominal somites relative to one another. This is crucial, as the swimming Movements of the crayfish consist of rapidly flexing the abdomen beneath the cephalothorax.

Fig. 83 Biramous appendage:

1,2 — two-segmented protopodite (basal part); 3 — dorsal branch (epipodite/exopodite); 4 — ventral branch (endopodite).

Place the crayfish on a sheet of paper or in the bottom of a dissecting tray, ventral side up. Examine the appendages of the head, thoracic, and abdominal segments: they differ quite significantly from one another and perform various Functions.

The ancestral structural type is the biramous appendage, which resembles the parapodium of annelid worms (Fig. 83).

The limbs of the middle abdominal segments exhibit this typical structure. Each consists of a basal part, the protopodite, from which two branches diverge: the endopodite (the inner branch closer to the body midline) and the exopodite (the outer branch) (Fig. 84). The protopodite, endopodite, and exopodite all consist of individual segments (podomeres). The appendages of all other body segments represent various degrees of modification of this basic plan: some parts are reduced, while others are strongly developed depending on their specific function. Examine the arrangement of the crayfish appendages, starting from the anterior end of the body.

Fig. 84. Appendages of the crayfish:

1 — antennule; 2 — antenna; 3 — mandibles; 4, 5 — First and Second pairs of maxillae; 6-8 — maxillipeds; 9-13 — walking legs; 14-19 — pleopods; 20 — uropods; 21 — gills.

1. Head:

✵ antennules (small antennae),

✵ antennae (large antennae),

✵ upper jaws, or mandibles,

✵ 1st pair of lower jaws (first maxillae),

✵ 2nd pair of lower jaws (second maxillae).

2. Thorax:

✵ 1st pair of maxillipeds,

✵ 2nd pair of maxillipeds,

✵ 3rd pair of maxillipeds,

✵ 1st pair of walking legs,

✵ 2nd pair of walking legs,

✵ 3rd pair of walking legs,

✵ 4th pair of walking legs,

✵ 5th pair of walking legs.

3. Abdomen:

✵ 1st pair of abdominal appendages,

✵ 2nd pair of abdominal appendages,

✵ 3rd pair of abdominal appendages,

✵ 4th pair of abdominal appendages,

✵ 5th pair of abdominal appendages,

✵ 6th pair of abdominal appendages.

The telson bears no appendages.

The antennules and antennae (smaller and larger antennae) function as Sensory Organs. They serve for tactile sensation and also bear microscopic cuticular hairs that act as chemoreceptors; through these, the crayfish can detect Changes in the composition and concentration of substances dissolved in water. Locate a small tubercle at the base of each antenna; the openings of the excretory glands are located at the apex of these tubercles. Insert the tip of a dissecting needle into the excretory opening.

Around the Mouth, which appears as a narrow longitudinal slit, are grouped appendages modified into mouthparts: the upper and lower jaws. The mandibles, which resemble wide serrated plates, play the primary role in biting and grinding food.

The antennae and jaws are located on the cephalic region. The following three pairs of jaw-feet—the maxillipeds—are the anterior appendages of the thoracic region. However, they have lost their locomotor function and instead assist in capturing food and bringing it to the mouth. The movement of the first and second pairs of maxillipeds generates a current of water through the branchial cavity. With the help of the endopodite of the 3rd pair, the antennules and eyes are cleaned of adhering foreign particles.

The walking legs—the appendages of the thoracic region—are uniramous and serve for locomotion. The 1st pair consists of the largest appendages of the crayfish. They are equipped with powerful chelae (claws), which the crayfish uses for capturing food and for defense. The 2nd and 3rd pairs of walking legs bear small chelae, while the 4th and 5th pairs lack them.

The abdominal appendages (pleopods) serve primarily for swimming. This function is particularly specialized in the appendages of the 6th segment; their branches are wide and flat. Together with the telson, they form a tail fan that assists the crayfish in swimming backwards (tail-first). In the male, the 1st and 2nd pairs of abdominal appendages are modified into a copulatory apparatus, which transfers spermatozoa to the female genital opening during copulation. In the female, the appendages of the 1st abdominal segment are reduced.

Locate the anus. It is situated on the telson and appears as a longitudinal slit.

Dissect the appendages from one side of the body and arrange them on a sheet of paper or cardboard in the same order as they appear on the crayfish's body.

Begin detaching the appendages starting with the antennae. Remove the antennule and antenna at their very base using forceps. Then, bending the 3rd pair of maxillipeds backward with a dissecting needle, detach them at the base with forceps, followed by the successive removal of the 1st and 2nd pairs of maxillipeds. Detach the lower jaws (maxillae) of the 2nd and 1st pairs. Examine The structure of each appendage as you remove it. After removing the lower jaws, proceed to detach the massive upper jaws (mandibles).

Gradually dissect the walking legs. Note that gills branch off from the 2nd and 3rd maxillipeds, as well as from the first four pairs of walking legs, and remain attached to the excised appendage. Detach the abdominal appendages (6 pairs in the male and 5 pairs in the female). Pay attention to the biramous STRUCTURE OF THE pleopods (except for the 1st and 2nd pairs in the male and the 1st pair in the female).

Draw and indicate the biramous structure of the antennae, the 3rd maxilliped, the 3rd pleopod, and the 6th pleopod.

Learn to distinguish males from females based on Sexual Dimorphism features:

✵ in the female, the abdomen is wider, whereas in the male it is narrower than the cephalothorax;

✵ the male genital openings are located at the Base of the 5th pair of walking legs, while in the female they are at the base of the 3rd pair; locate these openings by gently inserting a dissecting needle into them;

✵ in the male, the first two pairs of abdominal appendages are more developed than in the female and function as a copulatory organ.

The appendage of the 1st pair is shaped like a small tube. During copulation, the male releases sperm into this tube, where it rapidly thickens. Using the 2nd pair of abdominal appendages, the crayfish squeezes out these thickened sausage-shaped sperm masses and glues them between the female's abdominal appendages. When spawning, the female releases a fluid that dissolves the mucus holding the spermatozoa together; they are thus released and fertilize the eggs. The eggs adhere to the female's abdominal appendages; as she does this, the female curls her abdomen toward the cephalothorax. Fertilization and embryonic development take place within this "pocket".

Development proceeds without metamorphosis; the young crayfish hatching from the eggs differ from adults in their small size and disproportionate body parts. Young crayfish cling to their mother's abdominal appendages with their claws for a long time, remaining under her protection.

Dissection. For this practical, it is best to use live crayfish that have been anesthetized with chloroform prior to the class. If live material is unavailable, preserved crayfish stored in 70% ethanol can be used. Concurrently with the dissection, study the Internal Organs using a preserved specimen as well.

Place the crayfish dorsal side up. Bend the abdomen downward, insert the tip of the scissors beneath the carapace, and make two longitudinal incisions directed toward the larger antennae.

Make a transverse incision behind the eyes. Carefully remove the excised portion of the carapace, gently separating the underlying Tissues with a scalpel. Continue the lateral incisions along the abdomen down to the telson, connect them with a short transverse incision across the telson, and remove the cut pieces of the chitinous exoskeleton. Pin the crayfish dorsal side up to the bottom of the dissecting tray. Push the pins into the telson and the JOINTS OF THE claws.

Removal of the chitinous carapace exposes a bluish-red membrane—the hypodermis—and the very thin body wall. Remove them using forceps and scissors. Examine and sketch the overall arrangement of the internal organs. In the anterior body cavity (mixocoel), a voluminous Stomach is visible (Fig. 85), flanked by two powerful gastric Muscles. Gently press the anterior wall of The Stomach with forceps or a dissecting needle. Locate a pair of antennal, or green, glands (excretory organs) situated deep in the cephalothorax cavity anterior to the stomach, on either side of the Esophagus.

Behind the stomach lies the digestive gland (Liver), which is dull yellow in color (pale pink in live crayfish). The whitish, pentagonal sac in the posterior part of the body cavity is the heart. Beneath it, the gonad (Ovary or Testis) is visible. A straight tubular intestine extends from the stomach all the way to the telson, where it terminates at the anus.

Fig. 85 Internal Structure of the crayfish:

A — dissected crayfish; B — opened gastric mill of the crayfish; 1 — rostrum; 2 — gastric mill (stomach); 3 — digestive gland (liver); 4 — intestine; 5 — heart; 6, 7 — Arteries; 8 — ovary; 9 — ventral nerve cord; 10 — gills.

Examine the main Blood Vessels originating from the heart. For a more detailed study, refer to a special museum specimen of a crayfish with blood vessels injected with colored gelatin. The circulatory

System of the crayfish is open; blood circulates partly through vessels and partly through spaces between the internal organs. Thus, the blood also serves as coelomic fluid, which is why it is sometimes called hemolymph. It consists of a liquid plasma containing floating white Blood Cells, or leukocytes. In crayfish, it contains the pigment hemocyanin. The movement of the hemolymph is maintained by the continuous pulsation of the heart, which has thick muscular walls.

The heart is enclosed in a thin-walled pericardial sac, or Pericardium. It communicates with the pericardial cavity through three pairs of valve-bearing openings called ostia. Hemolymph leaves the heart via five anterior and two posterior arteries; their branches direct it to all PARTS OF THE body, where it pours into the lacunae—the spaces and clefts between the organs. Bathing the organs and tissues, the hemolymph supplies cells with nutrients and oxygen, while washing away carbon dioxide accumulated from cellular Respiration. This venous blood, rich in carbon dioxide and low in oxygen, enters the gills, where it releases carbon dioxide and is re-oxygenated, turning into arterial blood. Arterial blood flows via Veins into the pericardial sac, from which it is drawn into the heart through the ostia.

After familiarizing yourself with the general layout of the organs, make a drawing of the dissected crayfish and its organ systems, and then proceed to dissect and examine the individual organ systems in greater detail.

Dissect out and remove the heart, placing it with forceps onto a watch Glass in water. Use a hand lens to locate the heart's ostia. Examine the unpaired gonad (Fig. 86). The ovary can be identified by its brown or yellow color and numerous visible egg cells, which can be seen with the unaided eye ("caviar"). Two short oviducts extend from it, opening at the base of the 3rd pair of walking legs. The testis is white. A pair of long, convoluted vas deferens tubes extends from it, terminating in pores at the base of the 5th pair of walking legs. Remove the gonad and examine its shape.

Once the gonad is removed, the sections of the digestive tract (foregut, midgut, and hindgut) become visible. The mouth is located on the ventral side of the head and leads into a short esophagus. Using a dissecting needle, push the stomach aside and locate the esophagus.

Examine the stomach, which consists of two sections: an anterior, more voluminous cardiac (masticatory) stomach and a smaller posterior pyloric stomach ("strainer"). Cut open the wall of the cardiac stomach with scissors, rinse it with water from a pipet, and examine its inner surface, which is equipped with chitinous plates and folds designed for grinding food. The esophagus and stomach form the foregut.

Fig. 86 Reproductive Organs of the crayfish:

A — female; B — male; 1 — ovary; 2 — oviduct; 3 — female genital pore; 4 — testis; 5 — vas deferens; 6 — vas deferens opening.

Often, in the lateral folds of the cardiac stomach, an accumulation of calcium salts is found in the form of a bulky, white lump known as gastroliths ("crustacean stones"). This reserve of calcium salts is used during molting to impregnate the newly formed chitinous exoskeleton.

The midgut is short, flanked on either side by the digestive gland (liver), which consists of right and left lobes. The liver is formed by numerous closely packed tubules. It communicates with the midgut via two ducts, partially covering it. The midgut can be found by parting the liver lobes. The hindgut runs through the thickness of the abdominal musculature. Crayfish feed on various animal and plant matter, including carrion (dead fish, etc.). Food is captured and torn into pieces by the claws of the walking legs. The serrated edges of the jaws and maxillipeds grind the food into even smaller particles and feed them into the mouth. In the cardiac stomach, the food undergoes further comminution and mixing with digestive juices. In the pyloric stomach, the food is strained; the liquid fraction passes into the midgut (digestive gland).

The crayfish's digestive gland produces digestive Enzymes and partially digests and absorbs food. Reserve nutrients are also stored and accumulated within the liver cells. Undigested food residues are formed into fecal pellets in the hindgut and periodically expelled through the anus.

Remove the stomach. This will allow you to examine the excretory organs, which are represented by a pair of antennal glands located on the floor of the cephalothoracic cavity on either side of the esophagus. Due to their greenish color, they are sometimes called green glands. Dissect out these glands using forceps and examine them. Each gland consists of two parts. The lower part is formed by a bladder with a microscopically thin, convoluted tubule extending from it and emptying into the upper part, the reservoir. The short duct of the reservoir opens to the exterior—as observed during the external examination of the crayfish—at the base of the first segment of the 2nd pair of antennae.

The Central Nervous System consists of a well-developed supraesophageal ganglion (Brain), which is connected by commissures to the ventral nerve cord. The supraesophageal ganglion is situated in the head region anterior to the esophagus. Examine it using a hand lens. Nerves extend from it to the Sense Organs (eyes, antennae, and organs of balance and Hearing). The commissures loop around the esophagus and connect the supraesophageal ganglion to the first ganglion of the ventral nerve cord, the subesophageal ganglion.

Using scissors and forceps, remove the chitinous skeletal outgrowths on the floor of the cephalothoracic cavity and examine the ventral nerve cord. The nerve cord in the cephalothorax features five paired ganglia.