Phyla Cnidaria and Ctenophora, Basic Animal Development ...

Phyla Cnidaria and Ctenophora, Basic Animal Development & 

Introduction to Nervous Systems 5.1 

Lab #5 - Biological Sciences 102 – Animal Biology 

This lab is designed to explore the basic physiology and adaptations of the radiate animals in 

the Phyla Cnidaria and Ctenophora. We will also introduce nervous tissues and basic animal 

development with particular reference to the radiates. 

The two radiate phyla, Cnidaria (ny-dar'e-a) (= Coelenterata) and Ctenophora (te-nof'o-ra) are 

the most primitive of the eumetazoans (true multicellular animals). They are all radially (or 

sometimes biradially) symmetrical. Radially symmetrical animals have a body plan in which 

the parts are arranged concentrically around an oral-aboral (central) axis. These are the 

simplest animals that actually possess a true tissue level of organization. Generally, 

however, the tissues are not organized into organs with specialized functions. 

These phyla include several successful groups such as sea anemones, jellyfish, and corals, as 

well as the hydroids and comb jellies. Many cnidarians are brilliantly colored with many 

species forming the great tropical coral reefs that harbor the greatest diversity of life observed 

in the ocean 

Important Characteristics of Members of the Phyla Cnidaria and Ctenophora 

two embryological primary germ layers (ectoderm and endoderm) that are homologous to 

those of more complex metazoans 

internal space for digestion, the gastrovascular cavity, which lies along the polar axis and 

opens to the outside by a mouth 

some cnidarians have a skeleton (eg. coral), but in most radiates, fluid in the 

gastrovascular cavity serves as a simple form of hydrostatic skeleton 

although both cnidarians and ctenophores are grouped together as radiate phyla, they 

differ in important ways: 

o cnidarians have characteristic stinging organelles called nematocysts, usually 

absent in ctenophores 

o polymorphism- the presence in a species of more than one morphological kind 

of individual- is common in cnidarians but absent in ctenophores 

o ctenophores have distinctive adhesive cells called colloblasts on their tentacles 

and unique rows of ciliated comb plates not found in other phyla 

There are two main types of body form in cnidarians: 

1. polyp (hydroid) form, often sessile 

2. medusa (“jellyfish”) form, which is free-swimming 

In some groups of cnidarians, both polyp and medusa stages are 

found in their life cycle. These animals are therefore 

polymorphic. In others, such as sea anemones and corals, there 

is no medusa; in still others, such as the scyphozoans, or "true" 

jellyfish, the polyp stage is reduced or absent. In life cycles 

having both polyps and medusae, the juvenile polyp stage gives 

rise asexually to a medusa, which reproduces sexually. Both 

polyp and medusa have the diploid number of chromosomes, but 

the gametes are haploid. 

Discomedusae by Haeckel




Classification of the Radiate Animals 

Phylum Cnidaria 

Class Hydrozoa (hy-dro-zo'a) (Gr. hydra, water serpent, + zoon, animal). 

Both polyp and medusa stages represented, although one type may be 

suppressed; medusa with a velum; found in fresh and marine water. The 

hydroids. Examples: Hydra, Obelia, Gonionemus, Tubularia, Physalia 

Class Scyphozoa (sy-fo-zo'a) (Gr. skyphos, cup, + zoon, animal). Solitary; medusa stage 

emphasized; polyp reduced or absent; enlarged mesoglea; medusa without a velum. The true 

jellyfish. Examples: Aurelia, Rhizostoma, Cassiopeia 

Class Cubozoa (ku'bo-zo'a) (Gr. kybos, a cube, + zoon, animal). Solitary; polyp stage reduced; 

bellshaped medusae square in cross section, with a tentacle or group of tentacles at each 

corner; margin without velum but with velarium; all marine. Examples: Carybdea, Chironex 

Class Anthozoa (an-tho-zo'a) (Gr. anthos, flower, + zoon, animal). All polyps, no medusae; 

gastrovascular cavity subdivided by mesenteries (septa). 

Subclass Hexacorallia (hek-sa-ko-ral'e-a) (Gr. hex, six, + 

korallion, coral) (Zoantharia). Polyp with simple, unbranched 

tentacles; septal arrangement hexamerous; skeleton, when 

present, external. Sea anemones and stony corals. Examples: 

Metridium, Tealia, Astrangia 

Subclass Ceriantipatharia (se-re-an-tip' a-tha' ri-a) 

(combination of Ceriantharia and Antipatharia, from type 

genera). With simple, unbranched tentacles; mesenteries 

unpaired. Tube anemones and black or thorny corals. 

Examples: Cerianthus, Antipathes 

Subclass Octocorallia (ok'to-ko-ral'e-a) (L. octo, + Gr. korallion, coral) (Alcyonaria). Polyp 

with eight pinnate tentacles; septal arrangement octamerous. Soft and horny corals. 

Examples: Gorgonia, Renilla, Alcyonium 

A Ctenophore (Beroe) 

Hydra 

Cross section through a 

hexacorallian 

Phylum Ctenophora 

Class Tentaculata (ten-tak'yu-la'ta) (L. tentaculum, feeler, + ata, group suffix). 

With tentacles; tentacles may have sheaths into which they retract; some types 

flattened in oral-aboral axis for creeping; others compressed in tentacular 

plane to a band-like form; in some the comb plates may be confined to the 

larva. Examples: Pleurobrachia, Cestum. 

Class Nuda (nu-da) (L. nudus, naked). Without tentacles, but flattened in 

tentacular plane; wide mouth and pharynx; gastrovascular canals much 

branched. Example: Beroe




Basic Animal Life Cycles & Development (Embryology) 

On the chalkboard, your instructor will diagram a basic, generalized animal lifecycle to 

introduce the following terms related to animal life cycles and development. Your instructor 

will then review with you the basic life cycle of the cnidarians, Obelia sp. and Aurelia aurita as 

specific examples. 

EARLY ANIMAL DEVELOPMENT (see diagrams from board) 

sperm = the male gamete (haploid = N) 

ovum (egg) = the female gamete (haploid = N) 

fertilization = the species specific binding of the sperm to the egg and fusion of the 

sperm cell nucleus with the egg cell nucleus to create a diploid zygote 

zygote = fertilized egg (diploid = 2N) 

cleavage = the mitotic cell divisions of an animal zygote; the first cell divisions that 

occur to create a morula from the zygote 

morula = a solid ball of cells that froms from the mitotic divisions of the zygote 

blastula = the next embryonic stage that marks the end of early cell division during 

animal development; an embryo that look like a hollow ball of cells 

gastrula = the next embryonic stage resulting from the division of the early cells into 

three major tissue types during early animal development. The gastrula may have two 

(endoderm & ectoderm only) or three tissue layers (ectoderm, mesoderm, & endoderm) 

gastrulation = the process that leads to the formation of a gastrula 

ectoderm = the outer layer of three embryonic cell layers in a gastrula; forms the skin 

of the gastrula and gives rise to the epidermis and nervous system of the adult 

mesoderm = the middle layer of the three embryonic cell layers in a gastrula; gives rise 

to muscles, bones, the dermis of the skin, and many other organs in the adult 

endoderm = the innermost of three embryonic cell layers in a gastrula; forms the gut of 

the gastrula and gives rise to the innermost linings of the digestive tract and other 

hollow organs (e.g. lungs) in the adult 

larvae = an immature animal life stage that is significantly different from the adult and 

usually incapable of sexual reproduction 

metamorphosis = a drastic change in form during postembryonic development from 

the larval stage to another form, usually the adult 

adult = the sexually mature form of an animal 

Cleavage, the earliest stage in embryonic development, consists of a succession of regular 

mitotic cell divisions that partition the egg into a multitude of small cells clustered together. In 

lower animals, cleavage is so rapid that hundreds, sometimes thousands, of cells are produced 

in a matter of hours.




In animals such as sponges and cnidarians, cleavage is irregular and seemingly disorganized; 

egg cytoplasm is partitioned randomly into daughter cells of highly variable size and shape with 

no apparent relevance to future cell fates. As the metazoa evolved cleavage began to follow 

precise patterns and rhythms. In virtually all animal groups above the cnidarians, cleavage is 

regular; the egg cytoplasm is segregated into specific cells called blastomeres 

(Gr. blastos, bud, + meros, part) occupying discrete positions and having specific developmental 

fates. 

Patterns of regular cleavage depend greatly on amount and distribution of yolk in the egg. In 

eggs having a large amount of yolk, cleavage may be either complete (= holoblastic), as in 

amphibians, or incomplete (= meroblastic), as in birds and reptiles. In birds and reptiles with 

extreme telolecithal (Gr. telos, end, + lekithos, yolk) eggs, cleavage is restricted to a small disc 

of cytoplasm on the animal pole; this type of cleavage is called discoidal. The eggs of most 

insects follow another pattern of cleavage called superficial. In these the nuclei divide 

mitotically into hundreds or thousands of "free" nuclei, which later migrate to the egg surface. 

Only then do cleavage furrows form, rapidly partitioning the cytoplasm into a superficial layer 

of cells. 

In most invertebrates, eggs have little yolk (= isolecithal ["equal-yolk"]), and cleavage is 

complete (holoblastic) and equal. Two major kinds of holoblastic cleavage exist: spiral and 

radial (see text page 156, fig 8-7). The first two cleavages are the same in both kinds of eggs: 

the cleavage planes are along the animal-vegetal axis, producing a quartet of cells. At the third 

cleavage, however, these two patterns, spiral and radial, can be distinguished from each other 

by the geometric positioning of the cells. 

In radial cleavage, the third cleavage is perpendicular to the first two, yielding two quartets of 

cells, with the upper quartet lying directly on top of the lower. In spiral cleavage, the third 

cleavage planes are oblique to the polar axis and typically produce an upper quartet of smaller 

cells that come to lie between the furrows of the lower quartet of larger cells. 

There are other important differences between these two cleavage patterns. Spiral cleavage is 

typically mosaic, meaning that the embryo is constructed as a mosaic, with each cell fitting 

into its predetermined location in the larval body. If cells of the embryo are experimentally 

separated at this early stage, each cell will develop into partial or defective larvae because the 

developmental fate of each cell has already been determined. Spiral cleavage is found in 

several phyla, including annelids, many molluscs, some flatworms, and ribbon worms 

(nemerteans). All groups showing spiral cleavage belong to the grouping of animal phyla called 

the Protostomia, in which the embryonic blastopore forms the mouth. 

Early Embryonic Development - Cell Cleavages (mitosis) 

Radial cleavage is characteristically regulative 

because cell fate does not become fixed until after 

the first few cleavages. Radial cleavage is found in 

eggs of echinoderms and many chordates, 

especially protochordates, amphibians, and 

mammals. (As mentioned earlier, eggs of birds and 

reptiles, as well as many fishes, show discoidal 

cleavage.) All of these belong to the 

Deuterostomia, a group of phyla in which the 

mouth is formed from a secondary embryonic 

opening.




Introduction to Nervous Systems: Nerve Nets in Cnidarians 

The cnidarian nerve net is an excellent example of a simple, diffuse nervous system. The nerve 

cells form a plexus (a network) at the base of the epidermis and the gastrodermis. There are 

two interconnected nerve nets, one in each tissue layer. Axons (nerve processes) end on other 

nerve cells at synapses or neural junctions with sensory cells or effector organs. As seen in 

most animals, nerve impulses are transmitted from one cell to another by release of a 

neurotransmitter via synaptic vesicles in the presynaptic neuron. One way transmission 

between neurons in higher animals occurs is because of the relative activity of the protein 

channels for sodium and potassium in the axons that generate the electrical signals (action 

potentials). In addition, synaptic vesicles are only found in the nerve cell terminal (synaptic 

buton). Cnidarian nerve nets are comprised of cells that have synaptic vesicles on both 

sides and different ion channel properties allowing transmission across the synapse in either 

direction. Most nerve cells in the epidermis are multipolar (many processes) although some 

have bipolar neurons (two processes). In cnidarians, there can be both one-way and twoway 

synapses with other neurons. Cnidarian neurons lack the myelin sheaths which we 

will see in other animals. This myelin is a fatty acid membrane (in fact, a cell membrane) that 

will increase the speed of action potentials in larger, more complex animals. 

There is no “central nervous system” in cnidarians where a concentration and integration 

between many neurons occurs as seen in other animals. In other animals there is a cerebral 

ganglion or brain and dorsal or ventral nerve cord which controls the activity of other 

peripheral nerve cells. In cnidarians, there is some organization to the nerve net. The nerves 

are grouped in ring nerves in the medusae of hydrozoans and in the marginal sense organs of 

scyphozoan medusae. In schyphozoans there is a fast conducting nervous system to 

coordinate swimming movements and a slower conducting system that coordinates the activity 

of the tentacles. 

Sensory cells synapse with nerve cells that have junctions with epitheliomuscular cells and 

nematocysts. Collectively this combination of cells forms a neuromuscular system which is 

an important milestone in the evolution of nervous systems. In fact, versions of nerve nets 

(plexuses) coordinate the rhythmic contractions of the digestive systems of many other more 

complex invertebrate and vertebrate animals. 

Some types of sensory cells found scattered in the 

cnidarian epidermis: 

Chemoreceptors = detect molecules/chemicals in the 

environment (eg. prey) 

mechanoreceptors or tactile receptors = responds to touch or 

water movement 

Some types of effector cells found scattered in the 

cnidarian epidermis: 

cnidocytes = have nematocysts 

epitheliomuscular cells = for covering and muscular 

contraction 

Near the end of the lab, your instructor will describe the 

basic structure of a multipolar nerve cell (neuron). 

Be sure to copy and label the board diagram in your 

notes.




LAB PROCEDURE 

NAME: LAB SCORE: 

Class Hydrozoa 

Basic Body Plan 

While the dominant life stage of both Physalia and Velella exist as pneustonic (living at the airwater 

interface) polyp stage, Physalia is a floating colony of polyps exhibiting extensive 

polymorphism while the polyp stage of Velella is actually solitary. 

Observe the specimens and/or diagrams of the species listed below. 

Record the descriptive information requested at the end of the lab for each species. 

Aglaophenia latirostris (common name = Ostrich Plume Hydroid) 

Physalia physalia (common name = Portugeuese Man-of-War) 

Vellela velella (common name = By-the-Wind Sailor)




Using the preserved, prepared slides identify the following structures of Obelia sp. 

Below, draw and label the Obelia sp. (polyp colony) with hydranths and gonangia 

with clear labels for each of the structures listed below. 

Use the internet and textbook to assist you in your drawings. 

coenosarc 

perisarc 

gonangium 

o medusae in gonangium 

hydranth 

o hydrotheca 

o mouth 

o tentacles




Draw and CLEARLY label the Obelia sp. medusa from a prepared slide. 

Use the internet and textbook to assist you in your drawing. 

tentacles 

radial canals 

manubrium 

gonads 

How many gonads are observed in this species? 

Feeding Demonstration in a Hydrozoan 

Hydra, a Solitary Hydroid 



Order Hydroida 

Suborder Anthomedusae 

Genus Hydra, Pelmatohydra, or Chlorohydra 

Hydras are found in pools, quiet streams and ponds often on the underside of aquatic 

vegetation. Your instructor will prepare a slide of hydra for your observation. The hydra may 

be contracted at first and then extend. Small ciliated protozoans which are symbionts of 

hydras are sometimes seen gliding over the body and tentacles. 

Your instructor will add a drop of Daphnia or similar food to the slide. Some extracellular 

digestion occurs within the gastrovascular cavity via enzymes released by gland cells. Food 

particles are then engulfed by cells of the gastrodermis and digestion is completed 

intracellularly. Undigested food is regurgitated as there is no anus. 

Observe and briefly describe the feeding reaction of the hydra below:




Class Scyphozoa 

Basic Body Plans & Life Cycles 

Clearly label the stages of the life cycle of Aurelia aurita shown below. 

Use the internet and textbook to assist you in with the labels.




Look at the prepared slides of the following life stages of a life cycle for Aurelia aurita. 

Make drawings and clearly label them to complete the diagram below. 

Use the internet and textbook to assist you in your drawings. 

scyphistoma 

strobila 

ephyra




Class Cubozoa 

Observe the specimen and/or diagram of Carybdea sp. 

Record the descriptive information requested at the end of the lab for this species. 

Class Anthozoa 

Subclass Hexacorallia 

Observe the specimens and/or diagrams of the following species. 

Record the descriptive information requested at the end of the lab for these species. 

Balanophyllia elegans 

Note that the scleractinian hexacorallians (stony corals) synthesize a consolidated calcium 

carbonate skeleton while anemones do not. 

Anthopleura sola 

What accounts for the green color often present in this species? 

Feeding Demonstration in an Anthozoan 

As available, your instructor may feed a small piece of mussel to one of the anemones on 

display in the lab. 

Briefly describe the feeding behavior of the anemone: 

Cnidocytes and Examination of Nematocyst Discharge 

Cnidocytes are cells found in the epidermis between epitheliomuscular cells of cnidarians. 

Cnidocytes have nematocysts which are tiny capsules composed of material similar to chitin 

and contain a coiled tubular filament which is a continuation of the narrowed end of the 

capsule. This end of the cnidocyte is covered by the operculum. The base of the filament may 

have spines are barbs. The filament and barbs may contain a poison. The cnidocytes in 

anthozoans do not have the trigger-like cnidocil. The cnidocil is a modified cilium. In 

anthozoans a modified ciliary-like mechanoreceptor is involved in triggering nematocyst 

discharge.




Discharge of nematocysts involves the following events: 

1. molecules released from prey may sensitize the cnidocytes, but tactile stimulation causes 

the nematocysts to discharge 

2. discharge is due to tensional forces created by a very high osmotic pressure within the 

nematocyst 

3. when triggered the high osmotic pressure causes water to rush in to the capsule. 

3. the build of water (increased hydrostatic pressure) forces the filament out and it turns 

inside out at the same time 

4. the barbs are then exposed and, if present, poison is injected into the prey 

Nematocysts of most cnidarians are not harmful to humans, but the stings of Physalia and 

some true jellies are painful and can be dangerous – particularly if the individual is allergic to 

the organic poison. 

The instructor will remove the tip of a tentacle from Corynactis californica or similar species 

and prepare a slide by squashing the tip under a cover slip. 

Label the undischarged and sketch a discharged nematocyst below. 

UNDISCHARGED NEMATOCYST DISCHARGED NEMATOCYST 

Subclass Ceriantipatharia 

Order Ceriantharia (tube anemones) 

Observe the specimens and/or diagrams of the following species. 

Record the descriptive information requested at the end of the lab for these species. 

The available ceriantharian such as Pachycerianthus fimbriatus 

Subclass Octocorallia 

Under a dissecting scope, observe the polyp morphology of Lophogorgia chilensis or Renilla 

kollikeri to identify the unique features of an octocorallian polyp. 

Record the descriptive information requested at the end of the lab for these species.




For the live specimens available for observation in the lab, record the requested 

information. 

Be sure to include descriptions of any live members of the Phylum Ctenophora if 

they are available. 



Scientific name: Aglaophenia latirostris 

Common name: 

General dimensions of specimen: 

Color of specimen: 

Life stage present (observed): 

Solitary or colonial: 

Benthic, planktonic or pneustonic: 

Unique structures or features: 

Draw a simple sketch to remind you of the basic structure of this species and any unique 

characteristics observed. 

Notes & observations to help you remember and distinguish this group/species:






Scientific name: Physalia physalia 

Common name: 















Scientific name: Vellela velella 

Common name: 














Class Scyphozoa 

Scientific name: Aurelia aurita 

Common name: 














Class Cubozoa 

Scientific name: Carybdea sp. 

Common name: 
















Scientific name: Balanophyllia elegans or other hexacorallian 

Common name: 
















Scientific name: Anthopleura sola 

Common name: 















Subclass Ceriantipatharia 

Order Ceriantharia 

Scientific name: Pachycerianthus fimbriatus 

Common name: 
















Scientific name: Lophogorgia chilensis 

Common name: 
















Scientific name: Renilla kollikeri or other octocorallian 

Common name: 













Phylum Ctenophora (if available) 

Class: 

Scientific name: 

Common name: 













LABORATORY NOTES: 

Actiniae by Haeckel 

Ernst Haeckel (1834 – 1919) was an eminent German biologist, naturalist, philosopher, 

physician, professor and artist who discovered, described and named thousands of new 

species, mapped a genealogical tree relating all life forms, and coined many terms in biology, 

including phylum, phylogeny, ecology and the kingdom Protista. Haeckel promoted and 

popularized Charles Darwin's work in Germany and developed the controversial recapitulation 

theory ("ontogeny recapitulates phylogeny") claiming that an individual organism's biological 

development, or ontogeny, parallels and summarizes its species' entire evolutionary 

development, or phylogeny.




LABORATORY NOTES: 

Peromedusae by Haeckel 

Discomedusae by Haeckel 

Trachomedusae by Haeckel




LABORATORY NOTES:

Phyla Cnidaria and Ctenophora, Basic Animal Development ...

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