Phylum Cnidaria
The major feature of this group is alternation of generation, with the medusa and polyp
stage as the two adult forms. One of the questions for each representative will be
whether or not it possesses alternation of generation. Note the anatomical similarities in
polyps and medusae.
Another feature evident in Cnidaria is the polymorphism associated with some species,
especially those that have evolved a colonial strategy. Note the differences in form and
function of the zooids of colonial species.
Class Hydrozoa
What are the key features of this class? What is the dominant body plan? How is it
similar/different from other classes of Cnidaria? What is a craspedote medusa?
Order Hydroida
Hydra - polyp-only species
Obtain a live Hydra, place it in a watch glass and observe it under a dissecting scope.
Follow the observations suggested in the lab manual. Play around with the specimen to
observe how it responds to light, touch,…. Later, add a daphnid to the watch glass and
continue to observe as Hydra captures and comsumes its prey.
Slide - Whole mount, ls and xs of Hydra. These slides highlight the internal anatomy.
Note the features identified in Figures 3.4 and 3.5. NOTE: we will not look at the fine
cellular structure noted in Fig. 3.5, but you will be able to distinguish cnidocytes under
high power on the ls slide.
Obelia
You will want to read through the section on Pennaria to familiarize yourself with the
anatomical features of this colonial, polymorphic cnidarian. Refer to the anatomical
features of Figure 3.8A and B. What are these structures and what is their function?
Note differences in the terminology for gastrozooids and gonozooids. Figure 3.9
highlights the epidermal nature of the gonads.
Gonionemus
An example of a hydromedusa. How is it different from a scyphomedusa? Observe in a
watch glass under a dissecting scope addressing the features and questions in the lab
manual. Figure 3.11
Order Siphonophora
What are the features that distinguish members of this order from other Hydrozoa?
Physalia – Potuguese man-o-war (Figure 3.13)
A colonial polyp. An observational specimen is available. Note the different forms of
polyps (and function). What are other distinct features? Follow the observational
procedures of the lab manual.
Order Chondrophora
What are the features that distinguish members of this order from other Hydrozoa?
Velella – by-the-wind sailor.
Observational specimens are available.
polymorphism and structure of penmatophora (Figure 3.14)
Note
Class Scyphozoa
What are the key features of this class? What is the dominant body plan? How is it
similar/different from other classes of Cnidaria?
Aurelia detailed examination of a whole specimen. Slide series of scyphistoma, strobilia,
ephyra, and planula larva (Figure 3.15). Key features of the adult – radial canals, ring
canal, gonads, oral arms, rhopalia. Follow lab manual procedures
Class Anthozoa
Most diverse group of Cnidaria. What are the key features of this class? What is the
dominant body plan? How is it similar/different from other classes of Cnidaria? What
are septa and what is their significance?
Subclass Zonantharia
What are the distinguishing characteristics of this group? Why are anenomes and stony
corals placed in this subclass?
Metridium – a typical sea anemone
Get a preserved specimen for dissection. Observe the external anatomy (basal disc, oral
end, aboral end, tentacles, mouth, body column, siphonoglyph).
Follow the instructions on p. 68 for the dissection. Use Figure 3.20 to identify the
internal anatomy (wall of pharynx, septa, acontia, septal filament, siphonoglyph,
directives).
xs Slide – you will want to look at multiple slides to find a view through the pharynx
(Figure 3.19A) and below the pharynx (Figure 3.19B). Note epidermis, directives,
mesoglea, siphonoglyph, gastrodermis, pharynx, incomplete mesentaries, retractor
muscles.
Use high power magnification to observe the strcture of acontia.
cnidocytes and gland cells at high magnification.
You should see
Stony Corals
Observe the examples of stony corals “skeletons”, including the arrangement of
sclerosepta (Figure 3.21). Refer to the observational procedure (p. 69) as it relates to the
non-living part of stony corals.
Subclass Alcyonaria
Soft Corals. What distinguishes this from stony corals? From other Anthozoa? Note
specimens of Renilla, Gorgonia, and Tubipora on display. What are key features of their
gross external anatomy (you will only see polyps on Renilla).