Modeling Hydra Behavior Using Methods Founded in Behavior-Based Robotics

16 Chapter 5. HydraFigure 5.1: The cnidarian Hydra. Image courtesy of BioMedia Associates [7].lakes, and streams. It is most often found attached to some vegetation by the base of itstubular body 1 , where it feeds on small aquatic invertebrates, e.g. the multicellular arthropodDaphnia or the single-celled protozoan Euglena [24]. Hydra lives and reproducessexually and asexually, in the latter case by means of budding. Most cnidarians have afree swimming medusa stage. Hydra, however, does not, and hence it remains in its tubeshapedpolyp form throughout its life. Because of their radial symmetry, cnidarians arecommonly referred to as radiate animals.5.1.1 PhysiologyHydra is, like other cnidarians, designed around two layers of cell tissue: the outer ectodermlayer, and the inner endoderm layer. These two cell tissues are separated by agelatinous section, named the mesoglea. The endoderm layer surrounds the animal’s gut,which only opening is located at the hypostome 2 . Hydra does not possess a central nervoussystem (CNS), but rather its nerve cells are organized in a nerve net that extendsthrough the body, with a limited number of sensory inputs and motor outputs. Nerve cellsare spread throughout the ectoderm as well as the endoderm tissue, and consist of two celltypes: ganglion neurons 3 , and sensory neurons. However, it is believed that these neuronsof Hydra actually are multifuntional, thus each possessing the functions of sensory-,motor-, and inter-neurons [63]. Hydra has two types of effectors, or motor cells, namely(1) muscular cells of the endoderm and the ectoderm layer, respectively, and (2) nematocysts,which are stinging organells, located on the tentacles of the animal. The muscle1 Henceforth, the base of Hydra’s body will be referred to as its foot.2 Area at the top of the animal’s body, and the base of its tentacles.3 Interconnected neurons that either process sensory information or control motor outputs.