Protostome Animals: The Ecdysozoa
(Chapter 31)
Ok, we will now “shift gears” regarding our discussion of protostomes and consider
the ecdysozoans (be sure to review information contained in Figures 2 & 3).
Ecdysozoa contains protostomes whose growth is constrained by their possession
of a tough outer covering.
Thus, when ecdysozoans grow they first must shed their outer covering (Figure 4).
Once they are free, they expand their bodies and then reform their protective
outer layer.
This process of shedding the outer covering is called “ecdysis.”
At least 7 phyla belong to Ecdysozoa, including Nematoda, Kinorhyncha,
Nematomorpha, Priapula, Onychophora, Tardigrada, and Arthropoda.
In this lecture we will discuss the 2 major ecdysozoan phyla, namely, Nematoda and
Arthropoda.
1
,Phylum Nematoda (the nematodes or roundworms)
Nematoda (Figures 2, 3, 5) is a large phylum with some 25,000 named species and
a GREAT many more species awaiting discovery and description.
In fact, specialists estimate that the actual number of extant nematode species
may approach 1.2 million, illustrating that ours certainly is a wormy world!
Nematodes (Figure 5) are abundant and diverse in many marine, freshwater, and
terrestrial habitats and many (but not most) members of this phylum are parasites
of animals or plants.
In some habitats nematodes can be amazingly numerous.
For example, it’s been estimated that an average spade full of fertile soil may
contain a million nematodes.
Nematodes range in size from small microscopic species to species which are well
over 1-m long.
Nematodes are bilaterally symmetrical and unsegmented worms (Figure 6).
The nematode body is covered by a thick yet flexible cuticle that is periodically
molted as the worm develops/grows.
Although they appear to have arisen from ancestors possessing a coelom (Figure
2), nematodes possess a pseudocoel (Figure 7).
The pseudocoel serves as a hydrostatic skeleton and it gains its rigidity from being
fluid filled and pressurized.
2
, Just as a vertebrate’s muscles can “work” against bone to affect movement, the
nematode’s muscles can “work” against a body that is made rigid by the pressure
within its pseudocoel (Figure 7).
Nematodes only possess longitudinal muscles and these run along the length of the
worm.
The lack of a combination of both longitudinal and circular muscles is responsible
for the typical sigmoid, snake-like wriggling form of movement typically displayed
by nematodes… a form of movement that works well for burrowing species.
Like other pseudocoelomates, nematodes lack a defined circulatory system.
Instead, the role of the circulatory system is performed by the fluids that
circulate within the pseudocoel.
Nematodes have a complete one-way digestive tract, i.e., they possess a mouth and
anus (Figure 7).
The mouth may be equipped with piercing organs called “stylets.”
A muscular pharynx (Figure 7) is used to suck food into the mouth.
Reproduction in nematodes is typically sexual and the sexes are usually separate.
During the course of their development, nematodes typically pass through 4 larval
stages prior to becoming an adult.
While many nematodes are free-living, some of the parasitic forms are important
pathogens in plants and animals.
3
(Chapter 31)
Ok, we will now “shift gears” regarding our discussion of protostomes and consider
the ecdysozoans (be sure to review information contained in Figures 2 & 3).
Ecdysozoa contains protostomes whose growth is constrained by their possession
of a tough outer covering.
Thus, when ecdysozoans grow they first must shed their outer covering (Figure 4).
Once they are free, they expand their bodies and then reform their protective
outer layer.
This process of shedding the outer covering is called “ecdysis.”
At least 7 phyla belong to Ecdysozoa, including Nematoda, Kinorhyncha,
Nematomorpha, Priapula, Onychophora, Tardigrada, and Arthropoda.
In this lecture we will discuss the 2 major ecdysozoan phyla, namely, Nematoda and
Arthropoda.
1
,Phylum Nematoda (the nematodes or roundworms)
Nematoda (Figures 2, 3, 5) is a large phylum with some 25,000 named species and
a GREAT many more species awaiting discovery and description.
In fact, specialists estimate that the actual number of extant nematode species
may approach 1.2 million, illustrating that ours certainly is a wormy world!
Nematodes (Figure 5) are abundant and diverse in many marine, freshwater, and
terrestrial habitats and many (but not most) members of this phylum are parasites
of animals or plants.
In some habitats nematodes can be amazingly numerous.
For example, it’s been estimated that an average spade full of fertile soil may
contain a million nematodes.
Nematodes range in size from small microscopic species to species which are well
over 1-m long.
Nematodes are bilaterally symmetrical and unsegmented worms (Figure 6).
The nematode body is covered by a thick yet flexible cuticle that is periodically
molted as the worm develops/grows.
Although they appear to have arisen from ancestors possessing a coelom (Figure
2), nematodes possess a pseudocoel (Figure 7).
The pseudocoel serves as a hydrostatic skeleton and it gains its rigidity from being
fluid filled and pressurized.
2
, Just as a vertebrate’s muscles can “work” against bone to affect movement, the
nematode’s muscles can “work” against a body that is made rigid by the pressure
within its pseudocoel (Figure 7).
Nematodes only possess longitudinal muscles and these run along the length of the
worm.
The lack of a combination of both longitudinal and circular muscles is responsible
for the typical sigmoid, snake-like wriggling form of movement typically displayed
by nematodes… a form of movement that works well for burrowing species.
Like other pseudocoelomates, nematodes lack a defined circulatory system.
Instead, the role of the circulatory system is performed by the fluids that
circulate within the pseudocoel.
Nematodes have a complete one-way digestive tract, i.e., they possess a mouth and
anus (Figure 7).
The mouth may be equipped with piercing organs called “stylets.”
A muscular pharynx (Figure 7) is used to suck food into the mouth.
Reproduction in nematodes is typically sexual and the sexes are usually separate.
During the course of their development, nematodes typically pass through 4 larval
stages prior to becoming an adult.
While many nematodes are free-living, some of the parasitic forms are important
pathogens in plants and animals.
3
