Lecture 13. February 24, 2003 M. Martinez Introduction to animals, invertebrates

The Kingdom Animalia exhibits vast diversity in form. Within a phylum, animals show diversity on a theme, or bodyplan. Across multiple phla, animals show similar morphologies with similar lifestyles, indicating evolutionary convergence.

Animals are multicellular, heterotrophic eukaryotes that lack a cell wall. Greater than 95% of animal species are invertebrate, which is defined by not having a backbone. Invertebrates are not an evolutionary grouping, as they have nothing that unites them as a group. All 39 phyla of animals include invertebrates; only one phylum contains anything besides invertebrates. Only the subphylum Vertebrata of the phylum Chordata contains vertebrate animals, those with a backbone, those commonly recognized as animals by the general public. Urochordata and Cephalochordata, the other two subphyla of Chordata contain solely invertebrate animals.

Phylum Porifera, the sponges, consists of the oldest animals on earth. Sponges are benthic, sessile and aquatic. These are simple animals of cellular grade organization and show no overall symmetry. The choanocyte/collar cell is the diagnostic cell of this phylum. The flagellum of the choanocyte drives water through its collar and through the body of the sponge. Food particles such as bacteria are filtered from the water for food. Choanocytes also develop into sperm, are released by a currently-male sponge, caught by the choanocytes of a currently-female sponge, and delivered to an egg (sometimes formed by a choanocyte). A free-swimming larva is released into the water and will settle and metamorphose into an adult sponge.

Phylum Cnidaria, the jellies, is a group of simple sac-like animals that exhibit tissue-grade organization. They have two embryonic tissue layers, ectoderm and endoderm; thus they are called diploblastic. The middle layer is mostly jelly-like protein and water, not a living tissue. Most cnidarians show radial symmetry and alternation of generations between a sessile, benthic, asexual polyp and a pelagic, sexual medusa. They are all carnivores that capture prey with the stinging cells, cnidocytes, on their tentacles. Reef-building corals have only the polyp stage and have a second mode of feeding. They house symbiotic algal cells, zooxanthellae, in their tissues. The zooxanthellae photosynthesize and produce enough food for themselves and the coral. Distribution of reef-building corals is limited to shallow, clear waters, such as the tropics, where the symbionts can have sufficient access to sunlight.