The name Mesozoa (mes-o-zo´a) (Gr. mesos
, in the middle, + zoon
was coined by an early investigator
(van Beneden, 1876) who believed that
the group was a “missing link” between
protozoa and metazoa. These minute,
ciliated, wormlike animals represent an
extremely simple level of organization.
All mesozoans live as parasites in
marine invertebrates, and the majority
of them are only 0.5 to 7 mm in length.
Most are composed of only 20 to 30
cells arranged basically in two layers.
The layers are not homologous to the
germ layers of higher metazoans.
The two classes of mesozoans,
Rhombozoa and Orthonectida, differ
so much from each other that some
authorities place them in separate
Rhombozoans (Gr. rhombos
spinning top, + zoon
, animal) live in
kidneys of benthic cephalopods (bottom-
dwelling octopuses, cuttlefishes,
and squids). Adults, called vermiforms
(or nematogens), are long and
slender (Figure 12-1). Their inner,
reproductive cells give rise to vermiform
larvae that grow and then reproduce.
When a population becomes
crowded, reproductive cells of some
adults develop into gonadlike structures
producing male and female
gametes. Zygotes grow into minute
(0.04 mm) ciliated infusoriform larvae
(Figure 12-1B), quite unlike the parent.
These larvae are shed with host urine into the seawater. The next part
of the life cycle is unknown because
infusoriform larvae are not immediately
infective to a new host.
Two methods of reproduction by mesozoans. A, Asexual
development of vermiform larve from reproductive
cells in the axial
cell of the adult. B, Under crowded conditions in the host kidney,
reproductive cells develop
into gonads with gametes that produce
infusoriform dispersal larvae that emerge in the host urine.
A, Female and, B, male orthonectid
parasitizes such forms as
molluscs, annelids, and brittle stars.
The structure is a single layer of
an inner mass of sex cells.
Orthonectids (Gr. orthos
, swimming) (Figure 12-2)
parasitize a variety of invertebrates,
such as brittle stars, bivalve molluscs,
polychetes, and nemerteans. Their life
cycles involve sexual and asexual
phases, and the asexual stage is quite
different from that of rhombozoans.
It consists of a multinucleated mass
called a plasmodium
, which by division
ultimately gives rise to males and
Phylogeny of Mesozoans
There is still much to learn about these
mysterious little parasites, but probably
one of the most intriguing questions
is the place of mesozoans in the
evolutionary picture. Some investigators
believe they represent primitive
or degenerate flatworms and even
place them in phylum Platyhelminthes.
Others place them close to
some protozoa, possibly ciliates.
Whether metazoans and mesozoans
derived independently from protozoan
beginnings or whether mesozoans
are indeed degenerate flatworms
is still an enigma.