BIOL 2003 Lecture Notes - Lecture 6: Acoelomorpha, Statocyst, Bilateria
Intro to Bilateria
September 25, 2015
All bilaterians are triploblastic – three embryonic germ layers (ectoderm, endoderm and
mesoderm).
(diploblastic gastrula – triploblastic gastrula)
*Mesoderm allows animals to become more complex
Triploblasts = mesoderm = body cavity
• Acoelomate – densly packed mesoderm cells
• Pseudocoelomate – fluid filled cavity doesn’t connect or line organs
• Eucoelmate – mesoderm lines gut and organs while connecting them together (more
complex and larger organ systems)
Development of Cephalization – formation of a head
• Being bilateral leads to moving in one direction
• Forward movement gives anterior and posterior
• Development of sense organs at anterior end to sense environment = head first
• Mouth in head region
Example:
• Statocyst in acoelomorpha (ensure head first)
• Ocelli in tubellaria (planaria) – light sensing spots
Early Bilaterians
• Phylum: Acoelomorpha
o Marine
o Most less than 2 mm
o Statocyst for orientation
o No anus or gut
▪ Syncytial digestive vacuole
• Phylum: Platyhelminthes – flat animals
o Divided by lifestyle (non parasitic vs parasitic)
o Polyphyletic and paraphyletic class with parasitic properties
o *Wishy-washy bc paraphyletic/polyphyletic
o No synapomorphies (not related to phylogeny)
o Characteristics:
▪ Flat + worm
▪ 20,000 species
▪ Acoelomate, bilateral, vermiform
▪ Most are parasitic
▪ No anus (most groups)
▪ Flame cells (excretion and osmoregulation)
▪ Well developed reproduction – most hermaphrodites
▪ Four classes
o Generic Platyhelminthes
▪ Acoelomate body plan (parenchyma)
▪ Dorso-ventrally flattened
▪ No circulatory system = very branched out digestive system
o Flame Cells = earliest excretory system
▪ Filters fluid from inside body
▪ Removes metabolic waste
▪ Controls osmotic pressure in the body – retains important ions
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Document Summary
Statocyst in acoelomorpha (ensure head first: ocelli in tubellaria (planaria) light sensing spots. Early bilaterians: phylum: acoelomorpha, marine, most less than 2 mm, statocyst for orientation, no anus or gut, phylum: platyhelminthes flat animals. Syncytial digestive vacuole: divided by lifestyle (non parasitic vs parasitic, polyphyletic and paraphyletic class with parasitic properties, no synapomorphies (not related to phylogeny, characteristics: Flat + worm: 20,000 species, acoelomate, bilateral, vermiform, most are parasitic, no anus (most groups) Flame cells (excretion and osmoregulation: well developed reproduction most hermaphrodites. Four classes: generic platyhelminthes, acoelomate body plan (parenchyma, dorso-ventrally flattened, no circulatory system = very branched out digestive system, flame cells = earliest excretory system. Filters fluid from inside body: removes metabolic waste, controls osmotic pressure in the body retains important ions. Flame + tube cells = protonephridia: class: turbellaria, 4,500 species, mostly free living (marine, freshwater, terrestrial, can be small to large.
