LAB 9: INVERTEBRATE STRUCTURE AND FUNCTION

Introduction 

As you have now heard many times, the term “invertebrates” refers merely to a general category of exclusionary (negatively defined) animals rather than a monophyletic taxon (i.e., a valid systematic group including all descendants of a common ancestor). Instead of thinking of “invertebrates” as a unified group of animals, we’ll focus on three distinctly different phyla and examine important ways in which they differ, especially in their basic body plan. 

 The three animals we will dissect in this lab session are the crayfish (Cambarus), and the common sea star (Asterias or Pisaster), and the Squid. These are representatives, respectively, of the Phyla Arthropoda, Echinodermata, and Mollusca.

The Phylum Arthropoda is by far the most diverse and numerous of all animal phyla, with more than 750,000 known species (in fact, half of all known organisms). Arthropods are found in every marine, freshwater, and terrestrial habitat imaginable. Their success is largely due to their high degree of evolutionary adaptability and their great mobility.

All arthropods have a segmented body, covered by a chitinous exoskeleton, which is typically divided into three distinct parts (tagmata, or collections of segments [each a tagma): the head, abdomen, and thorax. Each part is usually subdivided into several segments bearing various jointed appendages. We’ll focus on a common crustacean, the crayfish, a freshwater relative of the lobster.  

The Phylum Echinodermata includes five major groups of benthic (bottom-dwelling) or burrowing animals:. All echinoderms are noted for their spiny protective skin, their five-part body plan, and the presence of numerous tiny tube feet, small appendages which are part of the coelom-derived water vascular system. The tube feet function in locomotion, feeding, and respiration, and the coelom is involved with circulatory, reparatory, and excretory activities.

The Phylum Mollusca is composed of about 85,000 species and comprises the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. Molluscs are highly diverse, not only in size and in anatomical structure, but also in behaviour and in habitat.

A striking feature of molluscs is the use of the same organ for multiple functions. For example: the heart and nephridia ("kidneys") are important parts of the reproductive system as well as the circulatory and excretory systems; in bivalves, the gills both "breathe" and produce a water current in the mantle cavity, which is important for excretion and reproduction. 

Pre-Lab Questions 

            1.         What do the names of these three phyla mean (how are they literally translated)?          

            2.         We are most closely related to which of these three phyla? Why?

            3.         What types of skeletal structures do these Phyla posses?

            4.         What type of digestive system (complete/incomplete) do these Phyla posses?

            5.         What is a water-vascular system?

            6.         What type of body symmetry do these Phyla posses?

            7.         What type of circulatory system do these Phyla posses?

            8.         What type of reproductive system do these Phyla posses?

            9.         What types of body cavities so these Phyla posses?

 Procedure 

We will consider the characteristics that are key in arthropod success, and in differentiating among arthropod groups: number/type of appendages, degree of external segmentation, types of sensory organs. Specifically, you will do a careful dissection of the three representative organisms and answer the questions (for each organism). How do they:  

                                                            Move

                                                            Reproduce

                                                            Eat

                                                            Respire

                                                            Sense their environment and See

                                                            Excrete

 There are a number of unique adaptations and structures in these taxa, be sure you can identify and characterize the function of at least the following for each group:

                        Arthropods                   cephalothorax, abdomen, walking legs, swimmerets, gills

                        Echinoderms                 tube feet, water vascular system, madreporite, gonads

                        Mollusca                       eye, funnel, gills branchial veins, mouth/beak, chromatopores

Materials 

Dissecting Trays

Probes, dull probes, scalpel, scissors

Razor blades

Dissecting pins

Colored pencils

Large crayfish model on board in 125 lab

Preserved sea cucumber

Preserved sea lily

Sand dollar and sea urchin shells

Preserved sea urchin

Preserved squid, crayfish, and starfish - one for each 2 students

Slides

            Crayfish saggital sections or xs 7-5

            Developmental larval crayfish

            Starfish arm xs 7-11

REFERENCES/SOURCES:

 Helms, D.R., C.W. Helms, R.J. Kosinski, and J.R. Cummings. 1998. Biology in the Laboratory, 3e. New York: W.H. Freeman.