LECTURE 06 - Spiralia: Platyhelminthes

Question 1

Describe the spiral cleavage of Spiralia?

Answer 1

• The first two cleavage divisions are equatorial and establish the four quadrants
• The third division is unequal, giving rise to 4 micromeres at the animal pole and 4 macromeres at the vegetal pole
• The plane of the mitotic spindle is twisted 45° to the perpendicular, so all 4 micromeres are twisted clockwise (to the right viewed from the animal pole) or anticlockwise (and left) and lie in the furrows between the macromeres
• The fourth subsequent divisions are likewise unequal and alternate right and left
• Cleavage is determinate: each cell in the embryo contributes to a specific tissue
• The result is a hollow ciliated blastula that develops into a prototroch larva

Question 2

What is the view on Platyhelminthes?

Answer 2

• The classical view is that Platyheltminthes are early-branching and represent the simple acoelomate body plan of the ancestor of Bilateria
• This view was overthrown by consistent molecular phylogenies, which located Platyhelminthes as a derived clade within Protostomia

Question 3

What is the anatomy of Mesostoma?

Answer 3

• Body surface
• Mesenchyme
• Digestive system
• Sense organs
• Nervous system
• Reproductive system
• Circulatory system
• Development

Question 4

Describe Mesostoma’s body surface.

Answer 4

• The outer barrier of the worm is the epidermis, a single cell layer of ectodermal origin, with thousands of beating cilia
• The cells bear elongate, hollow fluid-filled structures oriented at right angles to the body wall: these are the characteristic rhabdites of the Rhabditophora (one of the classes in Platyhelminthes)
• There is no cuticle or exoskeleton
• Underneath, an outer layer of circular muscle and an inner layer of longitudinal muscle that allows the body to change shape

Question 5

Describe Mesostoma’s digestive system.

Answer 5

• Mesostoma is an active predator that catches prey in two ways
  
  • By slime nets that entangle the prey
  • By active leaping onto the prey, wrapping the body around it, everting the pharynx and sucking in prey fluids and tissue
• The digestive system comprises mouth, pharynx and intestine
• The mouth is very extensible and can accommodate large prey
• The pharynx is an eversible muscular tube that grasps prey items
• Food is digested extracellularly in the blind, tubular intestine
• Solid indigestible waste is excreted through the mouth

Question 6

Describe Mesostoma’s eye

Answer 6

• Mesostoma has a pair of pigment-cup eyes each with two photoreceptors
• They cannot form images but can detect the presence and direction of light
• Other sense organs are : tactile cells with bristles; chemoreceptors: ciliated pits in head region; and, in some forms, statocysts

Question 7

Describe Mesostoma’s nervous system

Answer 7

• General features of the nervous system of Rhabditophora
  
  • A submuscular plexus at the border of the subepidermal muscles and the mesenchyme
  • Several (usually four) pairs of radiating longitudinal nerve cords, especially well-developed ventrally, with cross-connections
  • A cerebral ganglion (brain) anteriorly, connected directly or indirectly with the longitudinal nerve cords

Question 8

Describe Mesostoma’s excretory system

Answer 8

• The excretory system is based on flame cells communicating with ducts in the mesoderm that open as pores in the pharyngeal wall
• These cells are also responsible for osmoregulation

Question 9

Describe Mesostoma’s reproductive system.

Answer 9

• Almost all rhabditids are hermaphroditic with internal fertilization and outcrossing
• Gonads usually bounded from mesenchyme; variable shape and number
• FEMALE SYSTEM:
  
  • yolk held in separate yolk cells
  • one or many ovaries with one or two oviducts
  • seminal bursa to retain sperm
• MALE SYSTEM
  
  • one or two sperm ducts passing to seminal vesicle and penis bulb
  • penis papilla often armed with spine or stylet
• Copulation
  
  • mutual insertion of penis papilla into common gonopore
  • in some cases, hypodermic insemination directly into mesenchyme

Question 10

Why are flatworms either flat or small?

Answer 10

-There is neither a body cavity nor a circulatory system
- Hence, all gas exchange occurs across the surface of the body, which is why flatworms are either flat or small

Question 11

What are the two different types of eggs for Mesostoma?

Answer 11

• Subitaneous (rapidly-hatching): eggs produced in the summer
• Opaque resistant eggs produced in the fall

Question 12

What are Catenulida?

Answer 12

small freshwater worms forming chains of zooids
ciliated gastrodermis
often reproduces vegetatively

Question 13

What are Macrostomida?

Answer 13

Like Catenulids but possess rhabdites

Question 14

What are polycladida?

Answer 14

marine flatworms, usually littoral, often large

Question 15

What is the external anatomy of polyclad?

Answer 15

• The space between the dorsal and ventral epidermis is filled with parenchymal tissue which contains organs such as the highly branched gut and the reproductive system
• The parenchymal tissue is of mesodermal origin and holds a high number of secretory cells which discharge mucus and other compounds through epidermal pores

Question 16

What is the internal anatomy of polyclad?

Answer 16

• Polyclads have a “foldable” pharynx with longitudinal and a concentric muscle layers which can change its shape and pump fluid into the gut cavity
• The gut with its numerous lateral intestinal branches extends through most of the body and is ciliated throughout to circulate digestive products
• Polyclads are hermaphroditic
• Ovary and testes are extensive and generally distributed on dorsal and ventral sides of the body

Question 17

What are proseriata and rhabdocoela?

Answer 17

several groups of small aquatic worms

Question 18

What are Tricladida?

Answer 18

Aquatic or rarely terrestrial flatworms of moderate size

Question 19

What is the anatomy of Tricladida?

Answer 19

• gonopore
• penis
• genital chamber
• Yolk gland
• Pharynx
• oviduct
• gut
• ovary
• testis
• nerve cord
• eye
• brain

Question 20

What are Neoderma?

Answer 20

Ectoparasites or endoparasites, including major sources of human disease

Question 21

What is “Planaria” the common name for?

Answer 21

• “Planaria” is the common name given to flatworms in the non-monophyletic group “Turbellaria,” which includes all the non-parasitic Rhabditophora

Question 22

What is the shared derived character of the Neodermata?

Answer 22

The tegument: a syncytial layer surrounding the body

Question 23

What are the main groups of Neodermata?

Answer 23

the parasitic Platyhelminthes
- Monogenea: ectoparasites with a single host
- Trematoda: endoparasites with two (or more) hosts: flukes
- Cestoda: endoparasites with two or more hosts and strobilation: tapeworms

Question 24

Describe Monogenea.

Answer 24

• Monogenea is a group of small ectoparasitic worms mainly found on the skin and gills of fish
• They pass directly from one host individual to another of the same species

Question 25

Describe Trematoda.

Answer 25

• Trematodes are internal parasites with a complex life cycle whose final host is a vertebrate

Question 26

Describe Cestoda

Answer 26

• the worm is anchored to the gut wall of the host by the scolex
• The rest of the body is a flat tape, the strobila, consisting of identical segments called proglottids
• They are continually proliferated from the base of the scolex
• Each has independent digestive, excretory and reproductive systems
• The eggs develop within the proglottid and when it eventually reaches the end of the strobila and drops off, it passes out of the anus as essentially a bag of eggs that can then proceed to infect the intermediate host