The document provides an overview of Orthonectida, a class of rare wormlike parasites from marine invertebrates, detailing their biological and physical characteristics, reproductive methods, and ecological relationships. It highlights their unique features, such as a body without organs primarily made up of ciliated cells, and discusses their life cycle, including various stages like plasmodia and morulas. Additionally, the document touches on their phylogenetic position, conservation status, and lack of significance to humans.

1. 
ZOO-457 Principles of
Parasitology
Dr. Shamaila Irum

2. 
ORTHONECTIDA
Lecture # 10
Note: This lecture is with reference to
1. HOST & PARASITE RELATIONSHIP
2. PHYLOGENETIC POSITION

3. 
PHYLUM ORTHONECTIDA
 Class: Orthonectida
 Family: Rhopaluridae
 Genera: Rhopalura, Intoshia, Stoecharthrum, Ciliacincta
 Family: Pelmatosphaeridae
 Genus: Pelmatosphaera

4. 
ORTHONECTIDA :
 Orthonectid, any of a class (Orthonectid) of rare
wormlike parasites of various marine
invertebrates, including bivalve mollusks
and polychaeta annelids; they are usually included
in the phylum Mesozoic, a group regarded as
intermediate between protozoans (single-celled
animals) and metazoans (multicellular animals) in
organization.

5. 
What's in a Name?
 The name Orthonectida means "straight
swimming," but these animals usually swim in a
spiral motion.

6. 
 Microscopic and worm like animal and body
length rarely exceed from 300 micrometer .
 Body without organs and consists of an outer
layer of multi-ciliated cells which enclose an
inner mass of eggs and sperm cells.
 In some species the circular and lon­
gitudinal
muscles are present beneath the epidermis.
CHARACTERISTICS :

7. 
 Locomotion by cilia.
 Fertilization takes place outside the host.
 Parenchymal cells absent but some ex­
tracellular
matrix present.
 Most species show gonochoristic sexu­
ality.
 Depending on the species of Orthonectida, either
the sexes are separate or both male and female
reproductive organs are present in the same animal.

8. 

9. 
TAXONOMY :
 The orthonectids were originally described
in 1877 as a class, and placed as an order of
the phylum Mesozoa. Recent study shows
that orthonectids are quite different from
the rhombozoans, the other group in
Mesozoa.

10. 
PHYSICAL
CHARACTERISTICS :
 Orthonectida are parasites that live in the tissues of
sea-dwelling invertebrates.
 The body of an adult Orthonectida consists of a jacket
of body cells arranged in rings around an internal mass
 Some of these body cells have hair like fibers and
some do not.
 Contracting muscle cells in the sex organ run in the
lengthwise, circular, and oblique directions.

11. 
GEOGRAPHIC RANGE :
 They live in coastal regions , in japan & also
along the coast of united states .

12. 
HABITAT :
 They live in tissue of sea dwelling
invertebrates , such as
 Flat worms
 Segmented worms
 Mollusks
 In animals have soft bodies such as
 Sea urchin & sea cucumber .

13. 
DIET :
 Orthonectida absorb nutrients within the
host's cells.

14. 
ORTHONECTIDANS AND
PEOPLE:
 Orthonectidans have no known
importance to people.

15. 
CONSERVATION STATUS :
 Orthonectidans are not threatened or endangered.

16. 
PLASMODIUM STAGE
 A plasmodium stage lives in tissues and spaces of gonads
and genitorespiratory bursae of the ophiuroid Amphipholis
squamata.
 It may spread into the aboral side of the central disc,
around the digestive system, and into the arms.
 Developing host ova degenerate, with ultimate castration,
but male gonads usually are unaffected.
 The multinucleate plasmodia are usually male or female
but are sometimes hermaphroditic.

17. 
MORULAS
 Some nuclei are vegetative, whereas others are agametes
that divide to form balls of cells called morulas.
 Each morula differentiates into an adult male or female,
with a ciliated somatoderm of jacket cells and numerous
internal cells that become gametes.

18. 
BODY FORM
 Monoecious plasmodia that produce both male and female
offspring may represent the fusion of two separate, younger
plasmodia.
 Male ciliated forms are elongated and 90 μm to 130 μm long.
 Constrictions around the body divide it into a conical cap, a
middle portion, and a terminal portion.
 A genital pore, through which sperm escape, is located in one
of the constrictions.
 Jacket cells are arranged in rings around the body; the number
of rings and their arrangement are of taxonomic importance.

19. 
TYPES OF FEMALES
 There are two types of females in this species.
 One type is elongated, 235 μm to 260 μm long and 65 μm to
80 μm wide.
 whereas the other is ovoid, 125 μm to 140 μm long and 65
μm to 70 μm wide.
 Otherwise, the two forms are similar to each other and differ
from the male in lacking constrictions that divide the body
into zones.
 The female genital pore is located at about midbody.
 Oocytes are tightly packed in the center of the body.

20. 
REPRODUCTION
 Males and females emerge from plasmodia and escape from
the ophiuroid into the sea.
 There, tailed sperm somehow transfer to and penetrate
females, where they fertilize the ova.
 Within 24 hours of fertilization, the zygote has developed into
a multicellular, ciliated larva that is born through its mother’s
genital pore and enters the genital opening of a new host.
 It is not known whether a plasmodium is derived from an
entire ciliated larva or from certain of its cells or whether one
larva can propagate more than one plasmodium.

21. 
RELATIONSHIP WITH
HOST & PARASITE :
 Most dicyemids attach themselves loosely to the
lining of a cephalopod kidney by their anterior cilia
 They are easily dislodged and can swim about
freely in their host’s urine.
 The relationship appears to be a commensal one; no
pathogenic
 the membranes could fuse at various points and
form endocytotic vesicles, and “transmembrane”
was suggested by uptake of ferritin.

22. 
LIFE CYCLE :

23. 
 The Infusoriforms larvae do not have ruffle
membranes
 They derive their nutrients largely or entirely
from their hosts’ urine, whereas Infusoriforms
larvae must live for a period on stored food
molecules.
 Oxygen is very low or absent in a cephalopod’s
urine, so nematogens and rhombogens apparently
are obligate anaerobes.
 The organisms live longer in vitro under nitrogen
.

24. 
 Infusoriforms can live anaerobically only until
their glycogen supply is consumed.
 Host is cap-shaped, discoidal, or irregular
depending on the species.
 Hosts may harbor more than one species.

25. 
HOST-PARASITE RELATIONSHIPS
 Good ultra structural studies of both dicyemids and orthonectid are available.
 Most dicyemids attach themselves loosely to the lining of a cephalopod
kidney by their anterior cilia.
 They are easily dislodged and can swim about freely in their host’s urine.
 The relationship appears to be a commensal one; no pathogenic
consequences of the infection can be discerned.
 However, a few species have morphological adaptations for gripping renal
cell surfaces, and, on the parasites’ dislodgement, renal tissues show an
eroded appearance.
 The ruffle membrane surface of nematogens and rhombogens evidently is an
elaboration to facilitate uptake of nutrients.
 Ridley showed that the membranes could fuse at various points and form
endocytotic vesicles, and “transmembranosis” was suggested by uptake of
ferritin.
 The peripheral cells of infusoriform larvae do not have ruffle membranes but

26. 
RUFFLE MEMBRANE SURFACE
OF NEMATOGENS AND
RHOMBOGENS

27. 
Nematogens And Rhombogens:
 Clearly, nematogens and rhombogens must derive their nutrients largely or entirely from
their hosts’ urine, whereas infusoriform larvae must live for a period on stored food
molecules.
 Oxygen is very low or absent in a cephalopod’s urine, so nematogens and rhombogens
apparently are obligate anaerobes.
 The organisms live longer in vitro under nitrogen or even in the presence of cyanide than
when maintained in urine under air or in the absence of cyanide.
Infusoriform:
 Infusoriform can live an aerobically only until their glycogen supply is consumed.
Adult Orthonectid:
 Adult orthonectid, on the other hand, require aerobic conditions. Recent studies show that
calottes vary quite a bit among taxa, being cone- or cap-shaped, discoidal, or irregular
depending on the species.
Dicyemid Species
 Hosts may harbor more than one species, and when that happens, the two dicyemid species
usually have different calotte shapes, and based on infrapopulation and community
observations,
 Furuya and co-workers suggest that different dicyemid species cannot co-occur in an
individual host unless they both have the same calotte shape.

28. 
HOST PARASITE CYCLE :

29. 
PHYLOGENETIC POSITION
 The phylogenetic position of Mesozoa has been a matter of
considerable debate.
 The central issue is whether these parasites are an early divergence
from early metazoans, or are degenerate metazoans.
 Early taxonomists placed them between protozoa and sponges
because of their cilia, small size, and simple cellularity.
 Arguments also have been made for considering dicyemids to be
primitive or degenerate platyhelminthes.
 The ciliated larva is similar to a miracidium in some ways, and the
internal reproduction by agametes in nematogens and rhombogens
parallels similar processes in germinal sacs of digenetic trematodes.

30. 
MOLECULAR STUDIES
 Molecular and ultra structural studies provide strong
evidence for the “degenerate” metazoan hypothesis.
 For example, dicyemids contain a peptide sequence
characteristic of super phylum Lophotrochozoa (annelids,
nemertenes, platyhelminthes) that is not found in
superphylum Ecdysozoa (nematodes, arthropods) or in
super phylum Deuterostomia.

31. 
ULTRASTRUCTURAL RESEARCH
 Furthermore, ultra structural research reveals cell-to-cell
junctions typical of complex metazoans and not found in
Cnidarians.
 We still do not know the ancestral group, although Furuya
and co-workers suggest that dicyemids may be progenetic
larval forms of parasites that once lived in now-extinct
predatory marine vertebrates such as mosasaurs.

32. 
REFRENCES :
 https://www.encyclopedia.com/environment/encyclopedias-
almanacs-transcripts-and-maps/orthonectida-orthonectidans
 https://www.britannica.com/animal/orthonectid
 https://animals.jrank.org/pages/1468/Orthonectidans-
Orthonectida.html
 http://www.biologydiscussion.com/animals-2/mesozoans/
notes-on-mesozoans-with-diagram/32843
 https://wiki.kidzsearch.com/wiki/Orthonectida
 https://link.springer.com/article/10.1007/s13127-015-0246-
2?shared-article-renderer