Practical 2: Chordates
Table of Contents
1. Study of Protochordate Museum Specimens
This "spotting" exercise involves identifying key protochordate specimens.
| Subphylum | Specimen Example | Key Identification Points (Comments) |
|---|---|---|
| Hemichordata | Balanoglossus (Acorn Worm) | 1. Soft, elongated, worm-like body. 2. Body is divided into three distinct regions: an anterior proboscis (acorn-shaped), a short collar, and a long trunk. |
| Urochordata | Herdmania (Sea Squirt) | 1. Bag-like or "purse-like" body, attached to a substratum. 2. Body is covered by a leathery tunic or test. 3. Two siphons are visible: a branchial siphon (incurrent) and an atrial siphon (excurrent). |
| Cephalochordata | Amphioxus (Lancelet) | 1. Small, fish-like, translucent body, pointed at both ends. 2. Shows segmented muscle blocks (myotomes). 3. Anterior end has an oral hood with tentacles (oral cirri). |
2. Study of Protochordate Sections (Slides)
This exercise involves identifying key histological sections of *Balanoglossus* and *Amphioxus*.
| Specimen (Section) | Key Identification Points (Draw and Label) |
|---|---|
| Balanoglossus T.S. Proboscis | 1. Circular outline. 2. Epidermis on the outside. 3. A central proboscis coelom. 4. Presence of a glomerulus (excretory organ) and a central complex. |
| Balanoglossus T.S. Branchiogenital Region | 1. Shows the dorsal pharynx with gill slits. 2. Shows the "wings" of the coelom containing the gonads (testes or ovaries). 3. Ventral esophagus is present. |
| Amphioxus T.S. Pharyngeal Region | 1. A large, central pharynx with numerous diagonal gill bars. 2. Dorsally located dorsal nerve cord (hollow) and notochord (solid rod). 3. V-shaped myotomes (muscle segments). 4. Atrium space around the pharynx. |
| Amphioxus T.S. Intestinal Region | 1. A simple, oval-shaped intestine (midgut) in the center. 2. Notochord and dorsal nerve cord are present dorsally. 3. Myotomes are present. 4. No gill bars. |
| Amphioxus T.S. Caudal Region | 1. Section through the tail. 2. Shows the caudal fin. 3. Contains notochord, dorsal nerve cord, and myotomes, but no intestine or pharynx. |
3. Study of Vertebrate Museum Specimens
This is a "spotting" exercise for identifying specimens from the major vertebrate classes.
| Class | Specimen Example | Key Identification Points (Comments) |
|---|---|---|
| Agnatha | Petromyzon (Lamprey) | 1. Eel-like, scaleless body. 2. Jaws are absent. 3. Possesses a circular, suctorial mouth (buccal funnel) armed with horny teeth. 4. Shows 7 pairs of gill slits. |
| Chondrichthyes | Scoliodon (Shark) | 1. Cartilaginous fish with a spindle-shaped body. 2. Ventral, crescent-shaped mouth. 3. 5-7 pairs of gill slits, not covered by an operculum. 4. Skin is covered with placoid scales (tooth-like). 5. Heterocercal tail (unequal lobes). |
| Osteichthyes | Labeo (Rohu / Bony Fish) | 1. Bony fish with a spindle-shaped body. 2. Terminal mouth. 3. 4 pairs of gills covered by a bony flap called an operculum. 4. Skin is covered with cycloid/ctenoid scales. 5. Homocercal tail (equal lobes). |
| Amphibia | Rana (Frog) | 1. Soft, moist, glandular skin. 2. Body divided into head and trunk (neck and tail absent in adult). 3. Limbs are pentadactyl; hind limbs are large and webbed for jumping and swimming. |
| Reptilia | Calotes (Garden Lizard) | 1. Body covered with dry, horny epidermal scales. 2. Body divided into head, neck, trunk, and tail. 3. Claws are present on the digits. |
| Reptilia | Naja (Cobra) | 1. Limbless, snake-like body covered in scales. 2. Presence of a hood behind the head. 3. Poisonous (neurotoxic). |
| Aves | Columba (Pigeon) | 1. Body covered with feathers. 2. Forelimbs are modified into wings for flight. 3. Jaws are modified into a beak (teeth are absent). |
| Mammalia | Pteropus (Bat) | 1. Body covered with fur (hair). 2. Forelimbs are modified into wings for true flight; the "wing" is a thin membrane of skin (patagium) stretched between elongated fingers. 3. Presence of external ears (pinnae). |
4. Key for Identification of Poisonous and Non-poisonous Snakes
This exercise involves distinguishing between poisonous and non-poisonous snakes based on key morphological features.
| Feature | Poisonous Snake | Non-Poisonous Snake |
|---|---|---|
| Ventral Scales (Belly Scales) | Small and uniform OR Broad, but not full width (e.g., Sea Snakes). Exception: Vipers and Cobras/Kraits have broad ventrals that cover the full belly. | Almost always broad and cover the full width of the belly. |
| Head Scales | Usually small and irregular (e.g., Vipers). Exception: Cobras and Kraits have large head shields. | Usually large and regular (head shields), like in a Rat Snake. |
| Loreal Pit | Present in Pit Vipers (a heat-sensing pit between the eye and nostril). | Absent. |
| Tail Shape | Laterally compressed or "flat" like an oar (in Sea Snakes). | Usually cylindrical and tapering. |
| Fangs (if visible) | Presence of enlarged, grooved or hollow teeth (fangs) in the upper jaw. | All teeth are usually small and uniform. |
| Bite Mark | Two distinct fang marks, often with a row of smaller teeth. | A 'U' shaped series of small, uniform teeth marks. |
Exam Tip: In India, the "Big Four" poisonous snakes are: Cobra (hood), Krait (large hexagonal scales on back), Russell's Viper (triangular head, chain-like spots), and Saw-scaled Viper (small, triangular head, arrow mark).
5. Study of Birds (Specimens, Beaks, and Claws)
A. Six Common Birds
You may be asked to identify and state the order of 6 common birds. Examples:
- House Sparrow (*Passer domesticus*) - Order: Passeriformes
- House Crow (*Corvus splendens*) - Order: Passeriformes
- Pigeon (*Columba livia*) - Order: Columbiformes
- Parrot (*Psittacula krameri*) - Order: Psittaciformes
- Domestic Fowl (*Gallus gallus*) - Order: Galliformes
- Duck (*Anas sp.*) - Order: Anseriformes
B. Types of Beaks and Claws
Observe specimens or charts to understand how beak and claw shapes are adapted to diet and habitat.
Bird Beaks
| Beak Type | Shape | Adaptation (Diet) | Example |
|---|---|---|---|
| Grain-eating | Short, conical, strong. | Crushing seeds. | Sparrow, Pigeon |
| Fruit-eating | Hooked, sharp, strong. | Tearing fruits. | Parrot |
| Flesh-eating | Short, strong, sharp, hooked. | Tearing flesh. | Eagle, Vulture |
| Insect-eating | Thin, slender, pointed. | Picking insects. | Robin, Hoopoe |
| Filtering | Broad, flat, with filters. | Filtering mud for food. | Duck |
[Image of types of bird beaks]
Bird Claws (Feet)
| Claw Type | Shape | Adaptation (Habitat/Action) | Example |
|---|---|---|---|
| Perching | Three toes forward, one backward (slender). | Grasping branches. | Sparrow, Crow |
| Grasping (Raptorial) | Large, hooked, sharp talons. | Catching and killing prey. | Eagle, Hawk |
| Scratching | Strong, short, thick toes with blunt claws. | Scratching the ground for food. | Fowl, Chicken |
| Swimming | Toes are connected by a web of skin. | Paddling in water. | Duck, Goose |
| Wading | Very long, slender, spread-out toes. | Walking on soft mud or water plants. | Heron, Jacana |
[Image of types of bird claws]
6. Mounts from Fowl Head (Pecten and Weberian Ossicles)
Syllabus Note: The syllabus specifies "Mount of Weberian ossicles, pecten from Fowl head". The Pecten is a structure in the bird's eye. However, Weberian ossicles are a chain of small bones found in fish (like carp and catfish), connecting the swim bladder to the inner ear for sound amplification. They are not found in fowl. This is likely a syllabus error. You should prepare to identify the Pecten from a bird and (separately) the Weberian ossicles from a fish. Please clarify this with your instructor.
A. Pecten (from Fowl Eye)
- Principle: To observe the pecten, a unique structure in the bird eye.
- Observation: This is a prepared slide or dissected specimen. Look for a black, comb-like, pigmented structure that projects from the back of the eyeball into the vitreous humor.
- Function: Thought to provide oxygen and nutrients to the retina, which lacks blood vessels.
B. Weberian Ossicles (from Fish)
- Principle: To observe the chain of four small bones (ossicles) that connect the swim bladder to the auditory system in Ostariophysan fish.
- Observation: A delicate, articulated chain of four small, modified vertebrae.
- Identification Points (Draw and Label):
- Claustrum
- Scaphium
- Intercalarium
- Tripus (the largest, articulates with the swim bladder)