Chapter 9 – Multicellular and Tissue Levels of Organization

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I. Intro - Origins of Multicellularity

1. Multicellular life has been on earth for 550 million years, only ~10% of earth’s geological history
2. The evolutionary explosion that occurred in the 100 million yrs. prior to the Cambrian/Precambrian

1. all animal phyla in existence today came at the boundaries of those 2 eras

2. about 15 - 20 others that are now extinct also came into being at that time as well

3. most of the rest of the history of life has had much extinction compared to that time

3. colonial hypothesis – multicellularity began as dividing cells remained together, as do many colonial protests (fig. 9.3)
4. syncytial hypothesis – multicellularity evolved from large, multinucleated cells that developed internal plasma membranes
5. Both of these happen in different protests
6. Are animals polyphyletic or monophyletic? The nearly simultaneous appearance of all animal phyla in the early Cambrian makes it hard to tell

1. if animals are polyphyletic, more than one explanation of the origins of multicellularity is possible

2.  more than one body form could be ancestral
3.  however, impressive similarities in animal cell organization support monophyletic origin (eg asters in cell division, cell junctions are similar in all animal cells, most animals produce flagellated sperm and most animal cells use similar proteins to accomplish movement)

II. Phylum Porifera - Sponges

1. Primarily marine animals that consist of loosely organized cells; approx 9k spp, from < 1cm to > 1m
2. Characteristics of members of Phylum Porifera include:
   1.  asymmetrical or radial symmetry
   2.  3 types of cells - pinacocytes, mesenchyme cells (amoebocytes) and choanocytes
   3.  Central cavity or several branching chambers, thru which water flows for filter feeding
   4.  no tissues or organs
3. Cell types, Body wall, and Skeletons
   1.  sponge cells are specialized for particular functions (division of labor)
   a. pinacocytes - flat, thin cells that line the outer surface of a sponge. Pinacocytes may be      slightly contractile and help sponge change shape. Some pinacocytes specialized into porocytes, which regulate water circulation (fig. 9.5)
   b.  jelly like layer under pinacocytes is termed mesohyl. Mesenchyme cells are amoeboid, and move about in the mesohyl. Specialized for reproduction, transporting and storing food, secreting skeletal elements (spicules)
   c.  beneath mesenchyme, lining inner chambers are choanocytes - collar cells. Flagellated cells with ring of microvilli surrounding flagella. Microfilaments connect microvilli, forming a net that helps filter edible particles (Fig. 9.5)

1. spicules are formed by amoeboid cells
2. made of CaCO3 or silica
3. may take on a variety of shapes ( fig. 9.6)
4. alternatively, skeleton may be made of spongin, a fibrous protein made of collagen - dried beaten and washed to produce commercial sponges

2. methods of food filtration and circulation reflect body forms in the phylum. 3 types: (fig. 9.7)

a. ascon body form - simplest and least common. Vaselike form;

1. ostia are outer openings of porocytes and lead directly to chamber called spongocoel
2. choanocytes line spongocoel and their flagellar movements draw water into the spongocoel thru the ostia
3. water exits sponge thru osculum, single large opening at the top of the sponge

b. sycon body form - sponge wall appears folded

1. water enters thru dermal pores, which are openings of incurrent canals

2. pores in body walls open to radial canals, and radial canals lead to spongocoel
3. choanocytes line radial canals and beating of flagella moves water from ostia, thru incurrent and radial canals, to spongocoel and out the osculum.

c. leucon body forms have an extensively branched canal system.

1. Water enters the ostium and moves thru branched incurrent canals,

2.  incurrent canals lead to choanocyte lined chambers. Canals leading away from the chambers are called excurrent canals
3.  proliferation of chambers and canals has resulted in absence of spongocoel. Often there are multiple exit points for water leaving sponge

1, sponges feed on particles that range in size from .1 to 50 um.

a. bacteria
b.  microscopic algae
c.  protists
d.  other suspended particles

                    2.  important in reducing coastal turbidity

a. 1 leucon sponge, 1 cm in diameter and 10 cm high, filters 20 liters of water/day!

3.  a few sponges are carnivorous - catch small crustaceans (deep water) with spicule-covered filaments.

4.  feeding methods - choanocytes filter small suspended particles.

a. Water passes thru collar near base and moves into spongocoel at open end of collar

b. suspended food is trapped on collar and moved along microvilli to base of collar, where it is incorporated into a food vacuole

c.  lysozymal enzymes and pH changes digest particle in vacuole
d.  partly digested food passed to amoeboid cells, that distribute it.

5.  other feeding methods -

a. pinacocytes lining incurrent canals may phagocytize larger food particles. Sponges may also absorb nutrients in sea water thru active transport

6.  Sponges get rid of waste thru diffusion, since all cells are in close contact with water
7.  Sponges have no nerve cells for communication/coordination, but somehow choanocytes can cease activities more or less simultaneously, ceasing water circulation. chemical messages sent by ameboid cells is one possible method

1. certain choanocytes lose collars and flagella and undergo meiosis to form flagellated sperm
2.  other choanocytes may undergo meiosis and form eggs. Eggs retained in mesohyl of parent

3.  sperm cells exit one sponge by osculum and enter another with incurrent water. they are trapped by choanocytes and put in vacuoles.

4.sperm lose collar and flagella, become ameboid and transfer sperm to eggs

5. early development occurs in mesohyl, then a flagellated larva forms. Larva breaks free, free-swims for up to 2 days before settling to substrate and develops into adult form (Fig. 9.8)

6.some sponges form resistant capsules called gemmules, which contain masses of ameboid cells. gemmules can survive freezing and drying (9.8c,d) When favorable conditions return, ameboid cells stream out of tiny opening, and organize into a sponge

7. Some sponges can regenerate from an individual cut or broken apart.

III. Phylum Cnidaria

a. radial symmetry

b. diploblastic, tissue level organization

c. gelatinous mesoglea between epidermal and gastrodermal tissue layers

d.  gastrovascular cavity

e.  nervous system in form of a net
f.  specialized cells called cnidocytes used in defense, feeding, and attachment

1. diploblastic tissue organization - cells organize into tissues that can carry out more complex functions than individual cells; all cells derived from 2 embryological layers

a. cnidocysts may have spines to penetrate prey

b.  some have toxins that are injected to paralyze prey

c.  others have unarmed tubes that wrap around prey or substrate for attachment
d.  some have sticky secretions to anchor itself. 6 or more types of nematocysts may be found on one individual

1. polyp - usually asexual and sessile; attaches to substrate at base, column (cylindrical body form) is capped by a mouth surrounded by tentacles

2. medusa is dioecious and free swimming. shaped like inverted bowl, tentacles hang from rim. Mouth is centrally located facing downward, and medusa swims by pulsating body walls. More mesoglea in medusas than in polyps    

D.  Maintenance Functions

1. gastrovascular cavity functions in digestion , respiration, and discharge of gametes (reproduction); all substances for these processes enter and leave from the mouth

2.  food of most cnidarians consists of very small crustaceans, though some eat small fish
3.  nematocysts entangle and paralyze prey, cells contract to shorten tentacles bringing food to mouth
4.  as food enters gv cavity, gastrodermal gland cells secrete lubricating mucous and enzymes, which reduce food to soup. Digestion completed intracellularly in food vacuoles. undigested material is expelled thru mouth
5.  water provides support, via hydrostatic skeleton - fluid confined in cavity, contractile elements of body can use for leverage
6.  polyps use variety of locomotive forms - somersault from base to tentacles and back; inchworm movement; or glide slowly along substrate on their base; walk in tentacles
7.  medusae move by swimming and floating; water and wind currents responsible for most horizontal movements; contractile movements of bell expel water, driving medusae thru water
8.  cnidarians have most primitive nervous elements in animal kingdom; they have a 2 dimensional nerve net that conducts impulses around the body in response to local stimuli. Strength and type of response depend on strength of stimulus - weak stim - tentacle retracts; strong stimulus may cause entire polyp to withdraw
9.  all gases and nitrogenous wastes exchanged by diffusion bc large surface area to volume ratio

1. sperm and eggs are released into gv cavity or to the outside. In some cases, eggs stay in mom till fertilization; embryo enlarges to form a ciliated, free swimming larva called a planula. Planula attaches to substrate, interior cells split to form gv cavity and polyp develops

2. medusae nearly always form from budding from body wall of polyp and polyps form other polyps by budding. buds may detach, or remain attached to contribute to a colony.

F.  Class Hydrozoa - hydras - small relatively common cnidarians; most marine, but some are freshwater -characteristics: cnidocysts in epidermis; release sperm and eggs out of body

1, most hydrozoans have alternation of generations, but in some, medusa stage is lost; in others, polyp stage is very small

2.  most hydrozoans are colonial w/ some individuals specialized for feeding and others specialized for defense or reproduction e.g Obelia
3.  Gonionemus has a mostly medusa form, living in shallow marine waters.
4.  Hydra is a freshwater hydrozoan that hangs from underside of floating plants in streams and ponds - lacks medusa stage

H.  Class Anthozoa - anemones and corals - colonial or solitary, and lack medusae. Differ from hydrozoans bc sperm and eggs released into gv and expelled from there

4. Phylum Ctenophora - comb jellies - often bioluminescent; these contain 8 rows of ciliated bands for locomotion, and specialized adhesive cells called colloblasts for prey capture. Other characteristics are similar to Cnidarians.

Lesson Objectives and Study Questions Chapter 9.

1. Describe the 2 current hypotheses that explain the origins of animal multicellularity.

2. What are arguments for and against the hypothesis of monophyletic origin of animals?
3. What are 4 defining characteristics of organisms in the phylum porifera?
4. Why is radial symmetry an advantage in free floating or sessile animals?
5. Name and describe the 3 general body forms of sponge categorization (e.g. ascon, etc)
6. List and describe the specialized cells types possessed by sponges.
7. What types of support systems do sponges have?
8. On what do sponges feed, and how do they eat?
9. How do sponges reproduce? What is a gemmule?

10. What characteristics of Cnidaria are most important in distinguishing it from other phyla?

11. Name the classes in the phylum Cnidaria, and provide a common name for an animal in each.

12. how has dimorphism (alternation of generations) diverged and developed in different cnidarian classes?

13. How does a nematocyst discharge, and what functions may it have? How is it related to a cnidocyte?

14. Describe the nervous system of Cnidarians.
15. How do Cnidarians eat and digest their food?
16. How do they exchange gases and nitrogenous wastes?
17. How do Cnidarians reproduce?
18. How is the Phylum Ctenophora distinguished from Phylum Cnidaria? Provide a common name for a Ctenophoran.