KINGDOM ANIMALIA

PHYLUM CHORDATA, SUBPHYUM VERTEBRATA - VERTEBRATES

INTRODUCTION

You should remember from last week’s lab that the Phylum Chordata is divided into 3 subphyla: Urochordata (tunicates), Cephalochordata (lancelets), and Vertebrata (vertebrates). Though the 3 subphyla differ greatly in external appearance and habits, they share several specialized features: a notochord, a dorsal hollow nerve cord, a pharynx with gill slits, and a post-anal tail. Last week, we explored the characteristics of urochordates and cephalochordates, noting the presence of the characteristic chordate features. Today’s lab will cover the subphylum Vertebrata. In vertebrates, the characteristic chordate features are present at some stage of the life cycle, but they are often lost or obscured in adults.

In contrast to the filter-feeding tunicates and lancelets, vertebrates are specialized for an active lifestyle and the active pursuit of food. They are characterized by a well-developed brain and sense organs, a cranium protecting the brain and sense organs, and (except in hagfishes) a vertebral column. Many of the distinctive features of vertebrates derive from a unique embryonic tissue known as neural crest.

The subphylum Vertebrata includes the animals we commonly refer to as fishes, amphibians, reptiles, birds, and mammals. There is some disagreement about classification, but it is common to divide the vertebrates into 7 classes. Three classes of vertebrates are commonly called "fishes": Agnatha, Chondrichthyes, and Osteichthyes. They are similar in being aquatic, in carrying out gas exchange using gills, and in lacking limbs with digits. However, in many important ways they are quite different from one another. For example, the agnaths (hagfishes and lampreys) lack jaws and paired fins, features that are present in all other vertebrates, which are collectively known as gnathostomes. Also, the chondrichthyan fishes (sharks, skates, rays, and chimaeras) have an internal skeleton made of cartilage, whereas osteichthyan (bony) fishes have an internal skeleton made of bone.

The land vertebrates are collectively known as tetrapods, because they exhibit 4 limbs. Tetrapod limbs have separate digits and a basic limb structure with a single bone in the upper segment, two bones in the second segment, multiple bones in a wrist or ankle joint, and then a hand or foot. A few tetrapod groups, such as snakes, have secondarily lost limbs. Among living tetrapods, members of the class Amphibia are least specialized for life on land. They have moist, glandular skin, and they generally return to the water to reproduce, laying jelly-like eggs in the water and exhibiting an aquatic larval stage (such as a tadpole).

The other tetrapod groups, the classes Reptilia, Aves, and Mammalia, are collectively known as amniotes and are more fully adapted for life on land. They are characterized by laying amniotic eggs and they have thicker skin with a keratinized epidermis to minimize water loss. These features allow vertebrates to become completely independent of the water. Reptiles, including turtles, crocodilians, lizards, and snakes, are ectotherms, relying on the external environment as the major source of body heat, and their skin is covered with keratinaceous scales.

Birds and mammals evolved separately from early amniotes. They independently evolved as endotherms, which are able to control their body temperature largely by the internal, metabolic, production of heat. Going along with endothermy is the development of some form of insulation. Birds utilize feathers for insulation. They are highly modified for flight, with the forelimbs forming wings and elongate feathers forming a flight surface. However, they retain the amniote characteristic of laying shelled amniotic eggs. Mammals evolved hair as insulation, as well as mammary glands to feed their babies milk. Most mammals have secondarily modified the amniotic egg for internal development in a uterus.

Vertebrate classification:

Class Agnatha – jawless fishes (hagfishes and lampreys)

Class Chondrithyes – cartilaginous fishes (sharks, skates, rays, and chimaeras)

Class Osteichthyes – bony fishes (all other fishes)

Class Amphibia – amphibians (frogs, salamanders, and caecilians)

Class Reptilia – reptiles (turtles, crocodilians, lizards, and snakes)

Class Aves – birds

Class Mammalia – mammals

PROCEDURE

1. External morphology of the yellow perch

Examine a preserved specimen of a yellow perch from the supply. The body of the yellow perch is indistinctly divided into three regions, a head, trunk, and tail; note that there is no neck. The rear margin of the head is marked by the gill openings laterally and by the beginning of the scales dorsally. The tail is that portion of the body posterior to the anus; just posterior to the anus is a urogenital opening with a urogenital papilla.

Most of the head is unscaled and covered with soft epidermis. Beneath the epidermis are thin, flat bones that roof the skull. The mouth is terminal in position and is bounded above and below by the jaws. The upper jaw is formed by a narrow premaxilla and a triangular maxilla. The premaxillae can move freely, allowing the mouth to project forward. The most anterior and prominent of the lower jaw bones is the dentary. The premaxillae and dentaries of perch bear small teeth; feel these with your finger. The eyes are high on the head and lidless, and anterior to the eyes on each side are paired nares. The ears of fishes are internal, with no external openings.

The posterior margin of the side of the skull consists of a large flap, the operculum, behind which is a wide slit, the gill opening. The gills lie underneath the operculum, and if you lift it you can see them. Portions of the head and the entire trunk and tail are covered with thin, bony scales. Fishes have a special sensory system, the lateral line system, which detects water movements around the fish. You can identify the lateral line by looking for a longitudinal row of scales that are pierced by small openings for the individual lateral line organs.

Both median (midline) and paired (right and left) fins are present in the perch. The fins of bony fishes are supported by segmented and branching rays known as lepidotrichia. In addition, the perch has solid, unsegmented and unbranched fin spines that help deter predators. There are 3 median fins, the dorsal, anal, and caudal fins. In some fishes, the dorsal fin is divided into separate anterior spiny and posterior soft-rayed components. The anterior paired fins are the pectoral fins, and the more posterior ones are the pelvic fins.

Figure 1. External morphology of a yellow perch.

Here is a video of the external morphology of the perch.

2. External morphology of a frog

Examine a preserved specimen of a leopard frog from the supply. Note that the body is formed by head and trunk regions. Though frogs do have a single neck vertebra there is no distinct neck, and they also lack a tail. The front and hind limbs are unequal in size and used for different purposes. The forelimbs are important in absorbing shock and bracing the frog when landing after a jump. They consist of an upper arm, forearm, wrist, and hand. The hand has 4 digits, and claws and nails are absent (this is true for amphibians generally). During the breeding season, male anurans often develop a swollen nuptial pad at the base of the inner finger that they use to grip the female during amplexus, a mating embrace. The hind limbs are considerably larger and longer than the forelimbs, and are modified for swimming and jumping. They are divisible into a thigh, shank, ankle, and foot. The foot contains 5 elongated digits, connected by a thin skin webbing.

Note that the skin is smooth, scale-less, and glandular. There are two prominent dorsolateral folds extending down the back, posterior to the eyes. These folds are variable among frog species, and may be used in classification. Some types of frogs have small, wart-like structures on the back and hind legs, but these are not well developed in leopard frogs. The eyes are prominent, and generally protuberant, though they can be retracted into the orbits. The upper eyelid is poorly developed and has limited independent movement, but the lower lid is well developed, and can cover almost the entire eye. A portion of the lower lid is transparent or translucent, and can be moved separately over the eye; this is the nictitating membrane. Behind the eye is the tympanic membrane, or eardrum. This covers the cavity of the middle ear and aids in sound transmission. The relative size of the tympanic membrane varies with the species, and in some species is considerably larger in males than females. The external nares are located near the tip of the snout and contain valves, which can close off the nasal openings. Male frogs often have vocal sacs, which serve as resonating chambers for sound production.

Figure 2. External morphology of a frog. (From www.animalcorner.co.uk.)

3. Cat skeleton

We will look at a cat skeleton as a representative mammalian skeleton. It is common to divide the skeleton into the cranial and postcranial skeleton. A mammalian skull consists of a cranium and a mandible. The cranium is the main part of the skull housing the brain and sense organs, and has the upper jaw fused to it. The mandible, or lower jaw, is formed from a single bone, the dentary, on each side. The cranium consists of an expanded braincase and a rostrum. At the anterior end of the rostrum is the opening of the nasal passage, the external nares. Note also the zygomatic arch, or cheekbone, and the auditory bullae, bony capsules that protect the delicate structures of the middle and inner ear. Mammals all have 3 middle ear ossicles (the malleus, incus, and stapes) that help transmit vibrations from the eardrum to the inner ear, but these won’t be visible in the cat skull. Finally, the hyoid apparatus is a complex of several small bony elements that supports the tongue and larynx and provides a place for attachment of tongue and neck muscles.

The postcranial skeleton can be divided into axial and appendicular components. The axial skeleton includes the vertebral column, which is formed by a series of individual vertebrae and can be divided into 5 distinct regions: cervical, thoracic, lumbar, sacral, and caudal. Cervical vertebrae form the neck, and with very few exceptions all mammals have 7 cervical vertebrae. The first 2 cervicals – the atlas and the axis, respectively – are specialized for the support and motion of the head. The thoracic vertebrae are found in the chest region and are the only vertebrae to bear ribs. Lumbar vertebrae follow the thoracics and tend to be stout with large, anteriorly-directed processes. The sacral vertebrae (generally 3 - 5 in number) fuse to form the sacrum. The 2 sides of the pelvic girdle attach to the sacrum and anchor the pelvic girdle to the axial skeleton. The number of caudal vertebrae varies greatly from one group to another, but they are usually simpler in form than the other vertebrae. The sternum, or breastbone, is formed from a number of individual bony elements that sometimes fuse to form a single structure. The ribs run from thoracic vertebrae to the sternum; most ribs are connected to the sternum by costal cartilages.

The appendicular skeleton consists of the pectoral and pelvic girdles and limbs. The pectoral girdle is formed by a scapula and clavicle. The scapula is a flat, blade-like bone with no direct connection to the axial skeleton; it has a distinct spine running down the lateral surface. Primitively, the clavicle forms a direct and bony link between the sternum and the scapula, but this connection has been lost in cats and other mammals that are specialized for running. The forelimb contains the humerus, which forms a ball-and-socket joint with the scapula, the radius and ulna in the forearm; the wrist, which is formed by a set of bones known as carpals; and the metacarpals and phalanges, making up the hand and fingers, respectively.

Each side of the pelvic girdle is formed by the fusion of 3 bones, the ilium, ischium, and pubis. The ilium is the most dorsal of these, and articulates with the sacrum. The ischium and pubis are the posterior and anterior ventral bones, respectively. A large, round socket (known as the acetabulum) receives the ball on the end of the thigh bone, the femur, and allows great freedom of movement of the hind limb. At the knee joint is the patella, or kneecap. The lower hind limb contains two bones, the tibia on the medial side and the fibula laterally. The tibia is generally the larger of the two. The bones forming the ankle are the tarsals, the bones of the foot are the metatarsals, and the toe bones are the phalanges.

Figure 3. Cat skeleton. (From Leach, 1946, Functional Anatomy of the Mammal.)

4. Vertebrate diversity

On demonstration are examples of several vertebrate classes:

a) Agnaths: lamprey, hagfish

b) Chondrichthys: dogfish shark

c) Amphibians: frog, salamander, caecilian

d) Reptiles: turtle, lizard, snake

e) Birds: pigeon skeleton, pigeon

f) Mammals: mouse