Chapter 40, Problem 1TYK

Summary Introduction

Introduction:

Cephalization and bilateral symmetry of a body contribute to the evolutionary development of invertebrates. Cephalization is the formation of head at the frontal region of the body of organisms. The head consists of brain having ganglia and nerve cords. Ganglia is a group of nerve cells with specialized functions. The enlargement and clustering of ganglia lead to the development of brain.

Answer to Problem 1TYK

Correct answer:

Cephalopods.

Explanation of Solution

Justification/explanation for the correct answer:

Option (d) is cephalopods. Mollusks have neuron clusters and major nerves. Different mollusks have different degree of cephalization. The cephalopods groups of mollusks have cluster of ganglia, which gets fused into a complex lobe. Hence, option (d) is correct.

Explanation for the incorrect answer:

Option (a) is vertebrates. The cephalization occurs in invertebrates and the vertebrates are evolved from invertebrates. So, it is an incorrect option.

Option (b) is annelids. Annelids have a cerebral ganglion at the front of their body and a ventral nerve cord, which works as brain, whereas a true brain has a cluster of ganglia and two nerve cords. So, it is an incorrect option.

Option (c) is flatworms. The formation of ganglia, that is, clustering of nerve cells, first occurred in flatworms, which is a simple invertebrate and the enlargement of ganglia occurred later in complex invertebrates. So, it is an incorrect option.

Option (e) is mammals. Mammals come under vertebrate group, which have developed from invertebrates, and head formation occurred in invertebrates. So, it is an incorrect option.

Hence, options (a), (b), (c), and (e) are incorrect.

Conclusion

Therefore, it can be concluded that the process of cephalization (frontal head formation) was first seen in cephalopods.