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This scientific text explores the **phylogeny and morphology** of several invertebrate phyla: Acoelomorpha, Platyhelminthes (flatworms), Mesozoa, and Nemertea (ribbon worms). It **compares their body plans**, focusing on features like **bilateral symmetry, germ layers, and coeloms**, highlighting the **evolution of cephalization**. The text also **details the diverse lifestyles** of these animals, from free-living to parasitic forms, and examines their **unique adaptations** in nutrition, digestion, and reproduction. Finally, it **analyzes the classification** of these phyla, noting the ongoing debate about monophyletic groupings based on shared characteristics.
What body plan is characterized by having only one plane of symmetry that divides the body into mirror images?
Bilateral symmetry
The concentration of sense organs and nervous control centers at the head is known as:
Cephalization
What is the middle germ layer in triploblastic organisms called?
Mesoderm
Which of the following lacks a coelom, but is still considered acoelomate?
Triploblastic organisms
What is the tissue derived from mesoderm that fills the space between the epidermis and digestive cavity in acoelomates called?
Parenchyma
What is the process called when a solid band of mesoderm splits open to form a coelom?
Schizocoely
What characteristic is used to describe elongated, bilateral invertebrate animals without appendages, such as flatworms?
Worms
What is the digestive system like in some acoelomorphs?
A mouth leading to a mass of endodermally derived cells
What are the yolk-filled eggs produced by acoelomorphs called?
Endolecithal eggs
What is a defining morphological feature of acoelomorphs, regarding cell division?
Duet-spiral pattern of cleavage
What is the name of the phylum commonly called "flatworms"?
Platyhelminthes
What is a unique characteristic of parasitic Platyhelminthes?
Syncytial tegument (neodermis)
Which class within Platyhelminthes includes free-living flatworms?
Turbellaria
What is the function of rhabdites in the epidermis of turbellarians?
Form a protective mucous sheath
Which type of cell in dual-gland adhesive organs is responsible for detaching the animal from the substrate?
Releasing gland cells
What does it mean for a body covering to be "syncytial"?
It has many nuclei enclosed within a single cell membrane
What is the name for the syncytial covering found in adult parasitic flatworms?
Tegument (neodermis)
What is one proposed function of the syncytial tegument in parasites?
Avoid host immune response
How do tapeworms absorb nutrients?
Through their tegument
What is the function of the parenchyma cells found in flatworms?
Fill spaces between muscles and organs
How do planarians feed?
They extend their pharynx and suck up food
What do the intestinal secretions of planarians contain?
Proteolytic enzymes
.
How do trematodes and monogeneans obtain nutrition?
They graze on host cells, cellular debris, and body fluids
What is unique about the digestive system of cestodes (tapeworms)?
They have no digestive tract
What is the name given to the phylum of minute, ciliated, wormlike parasites?
Mesozoa
What are the two classes of Mesozoa?
Rhombozoa and Orthonectida
Where do rhombozoans typically live?
In the kidneys of benthic cephalopods
What is the multinucleated mass that gives rise to males and females in orthonectids called?
Plasmodium
What is the phylum of worms known for having a long proboscis?
Nemertea
What is a characteristic of the epidermis of nemerteans?
Ciliated and have many gland cells
