_Lab3.worms.09s - Biology 05B Spring Quarter 2009 Lab 3...

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Biology 05B – Spring Quarter 2009 Lab 3 – page 1 Animal Diversity II – The Lower Invertebrates (Worms). Contents: Topic Page 1. Introduction 1 2. Phylum Platyhelminthes – the Flatworms 2 3. Phylum Nematoda – the Roundworms 5 4. Phylum Annelida – the Segmented Worms 7 5. Summary 11 1. INTRODUCTION A. PERSPECTIVE Last week our study of animal diversity began with a look at two phyla that included the sponges and the cnidarians. You saw that the sponges were at a cellular grade of organization. As such, their “bodies” were comprised of a limited number of cell types that were specialized for a number of different functions. While this form of cellularization demonstrated a clear division of labor, there was no sophisticated coordination of efforts between these cell types that would allow the behaviors that we usually associate with animals. Thus they were limited to a simple, passive, filter- feeding lifestyle. In contrast, you saw that the cnidarians were much more advanced – they were able to respond to stimulus with conspicuous muscular activity. The basis for this advancement was that they had both nervous and muscular tissue and thus had attained a tissue grade of organization. This allowed them to assume a more active lifestyle characterized by being carnivorous predators. However, their ability to respond to stimulus and move was still quite limited in comparison to more evolved animals. These limitations are largely derived from their diploblastic organization which does not lead to the differentiation of the advanced forms of nervous and muscular tissue that are required for coordinated muscular activity. This week we will look at three phyla of animals that are united by their possession of three tissue layers; the ectoderm and endoderm with which you are already familiar, and a new layer called mesoderm. These animals are thus referred to as triploblastic . The possession of three embryonically derived tissue layers is if immense evolutionary importance because mesoderm gives rise to true muscle tissue. Additionally, more sophisticated forms of nervous tissue are derived from ectoderm in triploblastic animals. An important part of this week’s lab will be to observe the more coordinated movements that are made possible by the presence of these advanced tissues. In addition to the appearance of a new germ layer, other new structural features will also be observed in some of these phyla that contribute greatly to the life style of these animals. Among these are a complete gut with mouth and anus, aggregations of tissues all contributing to a common function (these functional units being referred to as organs ), groups of organs that form organ systems that perform complicated processes such as reproduction and digestion, body cavities that are distinct from the spongocoel and GVC and, segmentation of the body cavity. B.
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_Lab3.worms.09s - Biology 05B Spring Quarter 2009 Lab 3...

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