critical thinking on certain questions concerning comparative anatomy of the vertebrates

7/28/2019 Critical Thinking on Certain Questions concerning Comparative Anatomy of the Vertebrates

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Seth Andrew J. Salih October 1, 2012

COMPANA NO1 MS. Glorina P. Orozco

Critical Thinking Questions

Chapter 1

1. Given your knowledge of human anatomy, what

characteristics unique to chordates are present in humans?

How have they been modified?

Since humans are complex chordates, they must, at

least during the embryonic stage, exhibit the four-chordate

characteristics. They, at that point, must have a

notochord, an endostyle, a dorsal hollow CNS and a postanal

tail. After that, those characteristics have been modified.

The postanal tail can be seen as the coccyx, or the

tailbone. Some say that the coccyx is vestigial while some

say that its curved shape is used by the females to assist

the baby during birth. The notochord has been obliterated

during development; but in its place, is the bony centrum

supporting the vertebral column.

The dorsal hollow central nervous system has been

modified into a brain, with its many ventricles, and itsdownward extension, the spinal cord. In the pharynx, the

pharyngeal slits formed and closed again. The pharynx is

the site where the skeleton and musculature of the jaws,

the thyroid and parathyroid glands, middle ear cavity, and

the thymus gland arises.

2. The tissues and organs in the human body possess a

recognizable size, shape and position relative to other

structures. How would these features compare to other

mammals, to other amniotes, and other vertebrates?

Animal form and structure, and function are always

related. Structure fits function. Functions of organisms

are almost always related to their habitat and mode of

life. An example would be the exoskeleton. Humans have

skin, which perform numerous functions such as protecting


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the viscera, lubricating and moisturizing the outer

surface. Mammals mostly have a lot of hair, or fur to

protect them. Reptiles have cornified exoskeletons, while

fishes have scales.

3. What are the superior to inferior regions in the

human body, and how would they compare to the anterior to

posterior regions in a fish?

The longitudinal axis divides the body into superior

and inferior regions of the body. Parts that are superior

are nearer the head or also called cephalad. Parts that are

inferior are those toward the tail or also caudad. The

superior regions of humans are anterior in terms of fishes.

They are both cephalad and are towards the head.

Similarly, the inferior regions of humans are inferior in

terms of fishes. They are both caudad and are towards the

feet, or tail.

4. In reviewing the various figures in this chapter,

what developmental features in humans are similar to those

of other craniates even if they are not preserved in the

adult?

Of all characteristics of chordates, the 4

distinguishing characteristics of chordates are those that

are similar to other craniates that are not retained during

adulthood. They would be notochord, endostyle, dorsal

hollow CNS, and postanal tail. Other characteristics are 3-

part brain, cranium, paired external sense organs,

cartilage and the neural crest and its derivatives.

5. In the section on Other Craniate Characteristics,

what comparisons can be made between the human conditionand the patterns seen in other craniates for each of the

discussed systems? How general are human features?

For the integument, human skin is composed of the

dermis and a multilayered epidermis. Many varieties of

glands develop from the epidermis and empty on the surface.


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For other craniates, the epidermis consists of a variety of

horny appendages such as spines, reptilian scales,

feathers, hair, claws and hooves. For respiratory

mechanisms, humans respire by lungs, as well as other

terrestrial animals. Fishes, by gills and some amphibians

by their skin. In fishes, amphibians, and reptiles, the

coelom is partitioned into a pericardial cavity and

pleuroperitoneal cavity. In birds and mammals, we have a

pericardial, pleural cavities and an abdominal cavity. Most

craniates terminate their digestive tract in a cloaca. In

mammals and birds, we have the anus.

6. In comparing humans to other terrestrial craniates,

we see the presence of a neck in contrast to the condition

in fishes. What explanation can you give for the origin of

a neck in tetrapods?

Fishes have their eyes on the sides of their heads.

This position of the eyes allows the fish to see in front

and on the sides. Upon leaving water to live on land,

tetrapods developed their neck vertebrae, which lengthened

the neck and allow the animals to move their heads to a

greater degree. This allows them to look down at the

ground.

Chapter 3

1. What are the living protochordates? Why is

Hemichordata excluded from chordates?

The Hemichordates, Urochordates and the

Cephalochordates have living examples. For Hemichordata, an

example is Saccoglossus or Dolichoglossus. For Urochordata,examples of the tunicates are Molgula and Pandocia. An

example of Cephalochordata or amphioxus is Branchiostoma.

Hemichordata is excluded from the other chordates

because the hemichordates do not exactly have the 4-

chordate characteristics. They only have structures similar


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or homologous to those of chordates. The stomochord in

Saccoglossus is homologous to the notochord of the

chordates

2. What are the big fouror the synapomorphies for

Chordata?

Synapomorphies are derived features reflecting common

ancestry. The big four of the chordates are the: notochord,

dorsal hollow central nervous system, postanal tail and

endostyle.

3. What structures found in amphioxus are homologous

to human features? If they have been modified, then how?

In amphioxus, a single blind ending branches off from

the underside of the gut. It used to be thought that the

caecum might be homologous to the liver of vertebrates.

Similarly, the endostyle in protochordates is homologous to

the vertebrate thyroid gland. The anterior end of the nerve

cord contains regions homologous with the vertebrate

forebrain and hindbrain.

4. If cephalochordates are the sister group to

craniates, based on fossil evidence what is your estimatefor the time of origin for craniates (at least minimal or

youngest estimate)? How would you explain a lack of fossils

if they were not found in the time frame of your estimate?

The estimated period where craniates could possibly

originate would be the Cambrian period. If there would be a

lack of fossils, it is due to the environment dissolving

certain fossilizable parts.

5. Why use a larval lamprey as a model for thehypothetical craniate ancestor? Does this imply that the

parasitic adult lamprey is a good model for the lifestyle

of early craniates? If not, how has the adult been modified

from its common ancestor with other craniates?


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Lampreys are odd animals, with peculiar anatomy. They

have a long and special life cycle, with an extended

embryonic and larval development. Ammocoete larvae exhibit

craniate features that are not present in amphioxus, among

which are special sense organs and a craniate means of

eliminating excess metabolic wastes.

Lampreys are not primitive organisms but represent

highly specialized parasites. When comparing lampreys with

fossil relatives, it is clear that their parasitic

lifestyle is a derived character. There are isolated

populations of landlocked lampreys that do not metamorphose

into parasitic adults.

critical thinking on certain questions concerning comparative anatomy of the vertebrates

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