BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. DIVERSITY OF LIFE ON EARTH

2.8. Kingdom Animalia (animals)

2.8.1. Evolutionary Trends

Animals account for one-quarter of the eukaryotic kingdom (Fig. 2.4). All of them are multicellular, since unicellular Organism-like creatures belong to the kingdom Protoctista. Animals differ from plants in that they are heterotrophic rather than autotrophic. Animals differ from Fungi—which are also multicellular heterotrophic eukaryotes—in their mode of Nutrition. Fungi can be described as absorptive heterotrophs, whereas animals are ingestive heterotrophs. Fungi digest their food externally and then absorb the resulting Digestion products, whereas animal nutrition typically involves ingestion (taking food into the body) followed by internal digestion. Any undigested food is egested (expelled from the body). In the course of evolution, animals have developed a variety of feeding strategies, namely predation, herbivory, omnivory, and parasitism. While fungi literally grow on their food, animals often have to search for it. This requires the capacity for locomotion to move from one place to another, which in turn necessitates a highly developed Nervous system, Sense Organs, and effectors. Furthermore, locomotion in large animals requires Muscles and a Skeleton, which also serve a supportive function.

When studying animals, it is essential to consider the evolutionary trends that led to increasingly complex Levels of Organization in body Structure. Only one animal group, the Sponges (Fig. 2.46), lacks true Tissues (Table 2.12), whereas all other animals possess them. A tissue is defined as a group of Cells, often similar in structure and origin, that work together to perform a specialized function. This allows for The formation of various tissues, each serving its own specific role. The acquisition of a specialized function by a tissue is termed differentiation. This same principle operates at the subcellular level, where different Cell Organelles are specialized to carry out different Functions.

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Fig. 2.46. Breadcrumb sponge (Halichondria panicea).

Table 2.12. Main characteristics of the phylum Porifera (sponges)

Division of labor generally enhances operational efficiency. There are higher levels of organization beyond tissues. Several tissues working together form an organ, such as The Stomach; likewise, a group of interacting organs constitutes an organ system, such as the Digestive System. The combination of various Organ Systems makes up the organism as a whole.

Just as cellular activity is coordinated within a tissue, the functioning of organs and their systems must also be coordinated. Hormones and The Nervous System are responsible for this regulation. As we will see further, the evolutionary emergence of more complex tissues, organs, and organ systems was accompanied by major changes in body plans, ultimately necessitating The Development of transport systems, particularly the Circulatory system. Blood is a fluid tissue that circulates throughout the organism driven by the contractions of Blood Vessels or The Heart.

Below we will examine The most significant of these evolutionary pathways using Representatives of the major animal phyla.