BIOL 2325 Lecture Notes - Lecture 4: Dioecy, Osmoregulation, Mollusca
Document Summary
2 major evolutionary advances for mobile organisms: Cephalization(concentrating sense organs in the head region) Primary bilateral symmetry (body divided along one plane of symmetry) Acoelomorpha, platyzoa, and mesozoa are the simplest animals with bilateral symmetry. Have a saclike gut without anus or lack digestive cavity. Have radial arrangement of nerves and lack ganglia of a true brain. Most are free living but some are symbionts or parasites. 4 classes: turbellaria(nonparasitic), trematoda(parasitic), monogenea(parasitic), cestoda(parasitic tapeworms) Ciliated cellular epidermis resting on a basement membrane. Digestion-pharynx opens posteriorly inside mouth through which it extends. Dual-gland adhesive organs with 3 cell types. Secretions of viscid gland called fasten microbilla of anchor cells to substrate. Secretions of releasing gland cells is a quick chemical detach mechanisms. Simplest subepidermal nerve plexus(like cnidarian nerve net) Have endolecithal eggs and simple gut and pharynx. Glide over slime track secreted by marginal adhesive glands. Eggs attach to substrate via little stalk. Clamp onto external surfaces using a hooked opisthaptor.
