Hydra’s Ghost

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The biological terms used in the text are explained in the Notes.

Notes

1. Meta, “next”; stasis, “placement”; local or distant spread of a tumor generally assumed to be from a primary site to a secondary site. Shostak, “Vegetative Reproduction by Budding”; Shostak, “Hydra and Cancer.”

2. Shostak, “Hydra’s Complexity.”

3. Hydra reproduces asexually through budding. Buds, resembling the parent, form as out pockets, develop tentacles, mouth, body cylinder, and foot, detach from the parent, and under the right environmental conditions and feeding, go on to bud themselves. Evolutionary biology or evo-devo is devoted to revealing developmental mechanisms through comparisons (especially genomic) among embryos and through inferences drawn from ancestral forms.

4. Hybridization is generally defined as the process of mating organisms of different varieties or species, but here hybridization refers to synthetic organisms formed by fusions of somatic cells as well as reproductive cells (egg and sperm). Symbiogeny is the inter-organismic equivalent of Lynn Margulis’s concept of intracellular symbiogenesis or serial endosymbiosis (symbionticism). The symbiogeny hypothesis proposes that eukaryotic symbiotes in fused partnerships evolved into the variety of complex animals presently constituting the Metazoa. Shostak, “Symbiogeny and the Evolution of Tissues.”

5. Pääbo, Neanderthal Man.

6. Margulis, Origin of Eukaryotic Cells. Eukaryotes (true kernel [nucleus]) are unicellular and multicellular organisms comprising the protozoa, algae, plants, and animals. Their cells contain at least one nucleus, a membrane enclosed package of DNA (deoxyribonucleic acid) consisting of coding regions (known as genes) and non-coding regions.

7. Mitochondria are the cell’s “mighty-mites,” subcellular, membrane-enclosed powerhouses that produce most of the cell’s chemical energy in the form of ATP (adenosine triphosphate) through respiration (i.e., metabolizing oxygen). Organelles are the membrane bound small “organs” within eukaryotic cells performing tasks from synthesizing cellular products to detoxifying them, producing proteins to degrading them, importing extra-cellular materials and exporting them, synthesizing lipids and modifying them. Hydrogenosomes are cell organelles derived from mitochondria that metabolize hydrogen instead of oxygen. The “golgi apparatus” (named after Camillo Golgi) is one of the rare eponyms retained in biology’s lexicon (as a consequence of widespread use) following biology’s shedding most eponyms. The “dictyosome” (net body), the preferred term, is generally described as a stack of flattened membrane-bound vesicles or plates functioning in the storage, modification, sorting, and packaging of proteins destined for export. Centrioles are intracellular bodies typically occurring in peri-nuclear pairs and associated with the poles of dividing cells. Each centriole is composed mainly of the protein tubulin organized in nine groups of three tube-like microtubules. Cilia (plural; cilium: sing.; Latin: eyelash) are the motile and non-motile hair-like structures universally associated with animal cells and many unicellular organisms, conspicuously the ciliates. Resting on a centriole-like basal body, the core of a cilium consists of nine pairs of microtubules arranged around two central microtubules.

8. The “imago” (Latin, noun: image) is a fully developed (hence the image of a) reproductive insect. An “imaginal rudiment” is a specific part of a larva (e.g., a wing rudiment) that gives rise to a particular part of the adult (e.g., a wing).

9. Davidson, Peterson, and Cameron, “Origin of Bilaterian Body Plans”; Peterson, Cameron, and Davidson, “Set-aside Cells.”

10. Spiralia (aka Lophotrochozoa: crest/wheel animals) once reserved for mollusks and annelids now includes lophophorates such as rotifers and platyhelminthes. Spiralian micromeres are the embryo’s cells formed obliquely (hence spirally) at the beginning of development by the cleavage of the embryo’s four macromeres. The descendants of the micromeres form most of the larva’s outer layer (known as ectoderm). Shostak, “Symbiogeny and the Evolution of Tissues.”

11. Metazoa includes all multicellular animals (sometimes extended to sponges) containing differentiated eukaryotic cells, composite tissues, and complex organs. Cnidaria (Gr. Knidē, knettle) is a phylum of aquatic animals incorporating Anthozoa (anemones and corals), Scyphozoa (jellyfish), and Hydrozoa (e.g., Hydra). Cnidarian adults typically approach radial symmetry and always bear cnidocytes (stinging cells). “Anthozoan planula larvae” are products of sexual reproduction in corals and anemones. “Hydrozoan embryos” are the pre-planula products of sexual reproduction in hydrozoans. Chen et al., “Phosphatized Polar Lob-Forming Embryos.”

12. The “Cambrian explosion” takes its name from the geological stratum in which most of its fossils are found and refers to a period of relatively rapid diversification (if not an “explosion”) in animal lineages occurring 570 to 530 million years ago. Chen et al., “Precambrian Animal Diversity.”

13. Williamson, Larvae and Evolution; and Williamson, The Origins of Larvae.

14. Williamson, “Caterpillars Evolved from Onychophorans”; and Williamson, “Reply to Giribet.”

15. Haeckel, Riddle of the Universe.

16. Protista aka protoctista and protozoa: Typically unicellular eukaryotic amoebae, algae, and fungi sometimes ranked as a kingdom or subkingdom but generally considered far too heterogeneous for formal classification. Siddall et al., “The Demise of a Phylum of Protists.”

17. Shostak,” Myxozoa in Haeckel’s Shadow.”

18. Martínez and Bridge, “Hydra.”

19. Campbell, “Tissue Dynamics”; Shostak, “Growth in Hydra viridis.”

20. Cnidarian polyps are generally sedentary as opposed to swimming medusae or jellyfish. Hydra, for example, is s hydrozoan polyp with a “head” (mouth surrounded by tentacles) at one end, a cylindrical body proximally studded with buds, a stand or peduncle, and a foot or holdfast at the other end.

21. Shostak, “How Cnidaria Got Its Cnidocysts.” Biofilms are composed of surface-adhering bacteria typically embedded in a matrix; syncytia (also symplasms) are multinucleate agglomerates of fused cells; coenocytes are multinucleate masses formed by division of an original nucleus and its progeny.

22. Shostak, “Symbiogenetic Origins of Cnidaria,” 167–68.

23. Cnidocysts (nematocysts or cnidae) are the explosive cell organelles encapsulated in cnidocytes (aka nematoblasts, nematocytes or colloquially stinging cells) that release an ensnaring or stinging thread. Shostak, “A Symbiogenetic Theory,” 49–58.

24. Shostak and Kolluri, “Symbiogenetic Origins,” 1–29.

25. Hwang et al., “Evolutionary Emergence”; Shostak, “How Cnidaria Got Its Cnidocysts.”