Pseudocoelomates
Aschelminths
Achelminth Phyla
Gastrotricha
 Rotifera
 Kinorhyncha
 Nematoda
 Nematomorpha
 Acanthocephala
 Loricifera
 Priapulida
 Entoprocta

Pseudocoelomates
Define pseudocoelomate.
 formed directly from the cavity of the
blastula
 The cavity is small
 Mostly filled with intestine and oviducts
or testes.

Gastrotricha
Microscopic
 Marine and freshwater
species
 Common in lakes,
ponds and seashore
sands

Rotifera

Ciliated crown or corona at the anterior end
 Carnivorous or parasitic
 Most common in freshwater environments
 Body structure:
– Head has corona
– Trunk has ridged plates and spines
– Foot has toes for attachment.
Rotifera
Kinorhyncha
About 150 species
 Lives in marine
sediment
 Uses head as an
anchor, pulls body
after it.
 Sexes are separate
 Feed on algae and
bacteria

Nematodes
bilaterally symmetrical
 worm-like
 surrounded by a cuticle secreted by
epidermal cells
 longitudinal muscles only

Nervous System - Nematodes
Simple ring of nervous tissue around
pharynx
 dorsal and ventral nerve cords running
the length of the body

Nematode Movement
Contract longitudinal muscles
 High internal pressure causes the body
to flex
 Moves by thrashing back and forth
 No cilia or flagella

Excretory Systems
Some have specialized cells that
excrete nitrogenous wastes
 Others have canals
 Others have canals plus specialized
cells
 Nematodes do not have flame cells

Nematode Reproduction
Most nematodes are dioecious
 Males use special copulatory spines
 The sperm move by pseudopodia, like
amoebas

Nematode Diversity
Close to 500,000 species
 Some species are generalists
 Others are much more specialized 
– one species of nematode is known only
from felt coasters placed under beer mugs
in a few towns in Germany.
Nematode Lifestyles
Many free living
 Many also parasitic


Play critical ecological roles as
decomposers and predators on
microorganisms
Nematode-Caused Diseases
Roundworms - more than ½ the world's
humans
 Hookworms
 Trichinosis
 Pinworms infestations - extremely
common parasite in the United States

– can be transmitted from human to human
by eggs floating in household dust
Filariasis (elephantiasis)
 Onchocerciasis (river blindness).

Nematomorpha – Horsehair
Worms or Gordian Worms
Up to 1m long, but very slender animals
(1-3mm).
 Free-living as adults
 Often find adults in very clean streams

Juveniles are Arthropod Parasites
Acanthocephala - spiny headed
worms

2-host parasites
– Must have invertebrate host
Spiny protrusible proboscis
 About 1150 species
 Dioecious
 Both circular and longitudinal
muscles

Acanthocephalans - Nutrition
Nutrition by diffusion
 Proboscis attaches to host intestine
 Cause extensive damage to the
intestinal walls


Economic importance: Some forms
cause serious discomfort and ill-health
to domestic livestock
Excretion

Protonephridia lined with flame cells
Reproduction
Reproductive structures are contained
in strange ligament sacs.
 In males two testes are contained within
this sheath.

Loricifera
Discovered in 1974
 Dioecious
 Have a large brain
 Little else is known
about them.

Priapulida – The Penis Worms
Only 9 species
 All marine worms
 Found in colder
water
 Predaceous
 Fossils date back
to Middle
Cambrian.

Entoprocta (Bryozoa)
Ciliary feeding device - LOPHOPHORE
 Were known as Bryozoa (moss
animals)
 4000 species
 Few > 0.5 mm long
 All aquatic (marine and freshwater)

Ectoproct Feeding
Entoprocta

Most are colonial
 Lives in secreted exoskeleton (zoecium) that
may be:
–
–
–
–

Gelatinous
Chitinous
Stiffened with calcium
Impregnated with sand.
Some ectoprocts could be mistaken for
hydroids but their tentacles are ciliated.
Ectoproct
Colony