Chapter 11 - Pseudocoelomates

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Intro: The pseudocoelomates, also known as the aschelminths, are seven different animal phyla grouped together for convenience- they come from different evolutionary lines. They are not closely related on a phylogenetic tree, and the fossil record is meager.

1. 2 hypotheses for phylogeny (evolutionary relationships) -

1. they monophyletic, based on all share pseudocoelom, cuticle, muscular pharynx, adhesive glands
2. polyphyletic - do not share common ancestor, because of the absence of any single unique feature found in all groups - similarities may be result in convergent evolution, bc they are in similar environments
3. correct phylogeny may be somewhere in between the 2 - all share a few features, mentioned, and may share a common distant ancestor, but diverged very early.

1. aschelminths are the first animals to have distinct body cavity, but they lack linings and membranes (mesenteries) found in more complex animals.
2. as a result, organs lie free in the cavity, which is often filled with fluid or gelatinous substances
3. most aschelminths have complete digestive system, the acanthocephalans and nematomophs do not have a complete digestive system

1. complete digestive system allows for mechanical and chemical breakdown of food, absorption, and feces formation to occur sequentially.
2. most aschelminths also have a muscular pharynx for feeding.

1. some aschelminths may shed cuticle (ecdysis) in order to grow
2. beneath cuticle is epidermis which secretes cuticle
3. beneath epidermis is longitudinal muscle

10.  most are freshwater animals, only a few are marine
11.  nematomorphs, acanthocephalans and many nematodes are parasitic, but the rest are all free livers

2. Phylum Rotifera ("wheel bearers) derive name from ciliated organ, the corona on head.

1. Intro

1. cilia do not beat in unison, off set just a little, so it gives the impression of a spinning wheel
2. .1 - 3 mm in length, abundant in freshwater habitats
3. < 10% are marine
4. bodies have approximately 1000 cells (when numbers of cells in organs is consistent from animal to animal, this is called eutely
5. rotifers are usually solitary, free swimming animals, but some live between grains of sand
6. characteristics:

1. triploblastic, bilateral, unsegmented pseudocoelomate
2. complete digestive system
3. anterior end has corona
4. posterior end with toes and adhesive glands
5. well developed cuticle
6. protonephridia with flame cells
7. males generally reduced or absent - parthenogenesis common

B.  External features

1. cuticle secreted by epidermis, often thickens to form a lorica - provides protection and is mains supportive element, although fluid provides hydrostatic skeleton
2. epidermis is syncytial
3. head contains corona, mouth sensory organs and brain (see fig)

1. corona surrounds large ciliated area called buccal field

1. pedal glands secrete adhesives for temporary attachment to substratum
2. ducts on glands can open toes, also for attachment

C.  feeding and digestion

1. feed on small microorganisms and suspended organic particulates
2. cilia create a current of water that brings food to mouth
3. pharyxn contains jawlike structure called mastax, which grinds food
4. pharynx leads from mouth to esophagus, then to stomach.
5. glands secrete enzymes into stomach and pharynx.

1. extracellular digestion and absoprtion occur in stomach
2. in some species, cloacal bladder passes waste material to anus which releases digestive wastes, waste from protonephridia, and reproductive products to water
3. anus is located where foot joins trunk

D.  other organ systems

1. all visceral organs lie in pseudocoelom filled with fluid
2. protonephridia empty into cloacal bladder, regulate water in rotifers
3. gas exchange and more nitrogenous waste diffuse across body surfaces
4. nervous system composed of 2 lateral nerves and primitive brain (ganglionic)
5. sensory structures include numberous ciliary clusters and bristles on antennae and corona
6. 1 - 5 eyespots on head are photosensitive
   

E.  Reproduction and Development

1. some rotifers reproduce sexually, most parthenogenically; males are sporadic or nonexistent in some classes. Apparently, physiological condition of the female determines whether she will lay male or female eggs
   

A.  less than 1 mm long, elongate, bilateral worms found exclusively in marine environments

1. live in mud and sand
2. no external cilia or locomotor appendages, so burrow only for movement, thru mud and sand with snouts
3. there are about 150 know species
4. body composed of units called zonites
5. head, zonite 1, bears mouth, oral cone and spines
6. neck, zonite 2, has spines called scalids and plates called placids

1. head can be retracted into neck

    7.  each trunk zonite has pair of lateral spines, and one dorsal spine

    8.  body wall is cuticle, epidermis, and 2 pairs of muscle

    9.  complete digestive system. Kinorhynchs feed on diatoms, algae and organic matter

1. roundworms - may be some of the most numerous animals on earth - 5 billion may be in every acre of fertile soil

1. number of roundworm species estimates range from 10k to 500k.
2. feed on every conceivable source of organic material - rotting organic debris, living tissues of plants, animals, fungi etc.
3. may be microscopic to several meters long, many parasitic but most are free livers
4. nematodes lack cilia and often move by thrashing via longitudinal muscles
5. most common roundworm parasite of US is pinworm
6. also medically important are ascaris (800 million people worldwide thought to be infected, trichinella, filarial worms (cause elephantiasis, heartworm)

1. commonly called horsehair worms or Gordian worms

1. adults all free living, but larvae all parasitic of arthropods
2. found in all running or standing water
3. extremely long and threadlike with no distinct head
4. body wall has thick cuticle, epidermis, longitudinal muscle
5. dioecious, eggs deposited in water. small larva with proboscis hatches and must quickly enter an arthropod (e.g. beetle, cockroach) either by penetrating it or being eaten; sexual maturity only attained in free living adult form

1. typically, these are endoparasites of vertebrates, especially fishes.

1. 2 hosts required to complete life cycle; juveniles are parasites of crustaceans and insects
2. generally small (< 40 mm long), proboscis covered with recurved spines;
3. glycocalyx covers tegument and protects worm from enzymes and immune system defenses
4. eggs pass into water and must be eaten by an arthropod - cockroaches, grubs (Beetle larvae), or crustaceans - amphipods, etc.
5. larvae burrow into the body wall of an invertebrate, and when that animal is eaten by a vertebrate (mammal, fish or bird) it matures, becomes sexually reproducing adult, and attaches to the intestine

1. Loriciferans live in the spaces between marine gravel

1. small, bilateral worm with spiny head called an introvert, a thorax and an abdomen surround by a lorica (armorlike plate)
2. loriciferans can retract both head and thorax into lorica

1. small group of marine worms found only in cold waters

1. live in mud and sand of seafloor and eat small annelids and other invertebrates
2. range from 2 mm to 8 cm
3. anterior part of body is a introvert that they can withdraw into trunk

Lesson Objectives/study questions

1. What are 2 hypotheses that account for the grouping of 7 relatively different phyla into the aschelminths?
2. List at least 8 characteristics of the aschelminth group
3. Describe how the rotifers got their name
4. What type of habitat do most rotifers live in, and what do they eat? How do they eat?
5. Describe 7 characteristics of members of phylum Rotifera.
6. How is a cuticle different from a lorica in rotifers?
7. Describe the general characteristics of the head of a rotifer.
8. what structures are on the foot of a rotifer and what are they used for?
9. How is the pseudocoelom arranged (in terms of the organs and linings and fluids)?
10. What type of digestion does a rotifer have?
11. Be able to explain the ecological niches of the other 6 phyla