BSC 196 Lecture 17: BSC 196- Lecture 17
Document Summary
Their bodies are held together by structural proteins such as collagen. Nervous tissue and muscle tissue are unique, defining characteristics of animals. Tissues are groups of similar cells that act as a functional unit. Most animals reproduce sexually; diploid stage usually dominating the life cycle. After a sperm fertilizes an egg, the zygote undergoes rapid cell division called cleavage. Cleavage leads to formation of a multicellular, hollow blastula. The blastula undergoes gastrulation, forming a gastrula with different layers of embryonic tissues. Most animals have at least one larval stage. More than 1. 3 million animal species have been named to date; far more are estimated to exist. The common ancestor of all living animals likely lived between 700 and 770 million years ago. Morphological and molecular evidence points to a group of protists called choanoflagellates as the closest living relatives to animals. Early members of the animal fossil record include the ediacaran biota, which dates to about 560 million years ago.
