Acoelomates: Phylum Platyhelminthes and Nemertea - Biosciweb.net

Acoelomates: Phylum Platyhelminthes and Nemertea and
Pseudocoelomates: Phyla Nematoda and Rotifera & Parasitism 6.3
Lab #6 - Biological Sciences 102 – Animal Biology
The Pseudocoelomate Animals
All bilateral animal phyla except the acoelomates possess a body cavity belonging to one of
two types: (1) true coelom, in which a peritoneum (an epithelium of mesodermal origin)
covers both the inner surface of the body wall and the outer surface of the visceral
organs in the cavity, or (2) pseudocoel, a body cavity not entirely lined with peritoneum (one
layer of mesoderm attached to the ectoderm).
A body cavity of either type is an advantage because it provides room for organ development
and storage and allows some freedom of movement within the body. The cavity is often fluidfilled
and provides for a hydrostatic skeleton in those forms lacking a true skeleton.
There are nine pseudocoelomate phyla, of which phylum Nematoda is by far the largest. All
pseudocoelomate phyla are at the organ-system level of organization.
In general pseudocoelomates tend to be cylindrical in body form, to be unsegmented, and to
have a complete (mouth-to-anus) digestive tract (this is absent in acanthocephalans). The
epidermis is usually covered with a cuticle. There are both aquatic and terrestrial members,
and parasitism is fairly common.
Body Plan Features Retained by Pseudocoelomates (seen in previously studied phyla):
1. presence of true tissues
2. bilateral symmetry
3. triploblastic structure (endoderm, mesoderm and ectoderm)
4. some degree of cephalization = head with sensory organs
5. some organ systems (formed of true tissues) are present
Body Plan Features & Characteristics of Pseudocoelomates:
1. possess a pseudocoelom (mesoderm lines ectoderm tissue side only)
2. they are unsegmented
3. complete digestive tract/system (from mouth-to-anus)
4. epidermis is covered with a cuticle