4 Flatworms, Mesozoans, and Ribbon Worms

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

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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