Methodological guidelines on tasks
With a single continuous coelenteron or gastro-vascular cavity. With the exception of the Ctenophora all have nettle cells.
There are two cellular layers and a mesoglea.
Class 1. Hydrozoa.
Coelenteron simple, without septa. Gonads usually ectodermal. Fully formed medusae have a velum.
Class 2. Scyphozoa.
Body-wall of polyp thrown into four ridges which project into the coelenteron. Medusae without velum and with gastric tentacles. Medusoid form predominating.
Class 3. Anthozoa.
With a stomodaeum, and with mesenteries ex¬tending into the coelenteron. Fixed forms.
HYDRA. (Fresh-water Polyp.)
Hydra, the only common fresh-water coelenterate, is frequently found in jars of water taken from quiet pools or sluggish streams that contain lily-pads, decaying leaves, and other vegetable matter. The animals may frequently be found by examining the surfaces of submerged leaves, but it is usually better to allow such material to stand in glass jars for a day or two, as the animals then tend to collect on the lighter sides of the vessels. They are easily kept in balanced aquaria.
Examine specimens in an aquarium and find what you can about their mode of life. Do they form colonies?
Place a specimen in a watch-glass of water and examine it with a lens
What is its shape and color? Is it attached? If so, by what part of the body? Notice the circlet of tentacles. How many are there? Compare notes with others and see if all have the same number How are they placed?
Does the Hydra move its body or tentacles? Is it sensitive? How do you know?
Examine with a low power of the microscope and review the above points. You may also be able to see the mouth around which the tentacles are arranged.
Make two drawings, one showing the animal expanded and the other contracted.
Place your specimen on a slide under a cover-glass that is supported by the edge of another cover-glass, so it can be examined with a high power. Be careful not to crush it. Notice: The outer layer, ectoderm. What is its color? Is it continuous over the whole outer surface? Does it vary in thickness? Are the cells of which it is composed apparently all alike?
The inner layer, endoderm. What is its color? If color is present, is it evenly diffused or is it collected in special bodies? Are the cells of which the endoderm is composed apparently all alike? Do they differ in appearances from those of the ectoderm other than in color? If the specimen is not deeply colored, look for flagella moving in the internal cavity.
Examine the ectoderm of the tentacles carefully and notice that each of the large, rounded, clear cells, the nematocysts, shows a rather indefinite streak running from its outer end, back into the interior. See if you can find the trigger (cnidocil) on any of these cells.
Draw a portion of a tentacle showing the distribution of the nematocysts.
Place your specimen under the low power of the microscope, carefully run in a drop of saffranin, and see if any of the nematocysts are discharged when the saffranin touches them.
Examine with a high power and notice the appearance of the thread. Notice the change in the shape of the nematocysts that have discharged. See if you can find two kinds.
Make an enlarged drawing of an exploded nematocyst.
Examine prepared transverse sections of Hydra. Notice that the body is composed of two layers of cells, between which is an almost structureless thin layer Do the cells of the two layers differ in size, shape, and structure? Do you find more than one kind of cell in each or either of these layers? Where are they? What are they?
Make a careful drawing of the section showing the arrangement as you see it.
Examine longitudinal sections, for differences in the character of the ectoderm and endoderm in different parts of the body.
Reproduction Examine living specimens in a watch-glass of water for bud formation and for sexual organs Spermarics are just beneath the tentacles, ovaries, lower down; buds may be found at different levels. What layers of cells is involved in the formation of each of these?
Eggs are not formed at all seasons of the year and vary greatly in appearance according to their stage of development.
Make drawings of the stages of reproduction that you find.
This form is one of the common jelly-fishes, and is found floating freely in the water. It is frequently washed up on shore. To be appreciated these medusas should be seen as they occur at the surface of the sea, before they have been handled or injured. Frequently vast numbers may be seen together, all gently pulsating and thus keeping near the surface. The movement is very different from that of most hydrozoan medusa, being very deliberate and graceful
If living material is offered, study the method of locomotion and compare it with the locomotion of Gonioncurus. Like the latter, the discoid animal presents ex-umbrellar (aboral) and sub-umbrellar (oral) surfaces, but the edges of the disk are indented, fringed with very numerous short tentacles, and a velum is wanting. What difference does the velum make in locomotion?
The ex-umbrellar surface presents little of interest. In the live specimens, however, prove that the animal is sensitive over this area as elsewhere.
Preserved and hardened material is better than living for the study of the rest of the anatomy of this form. With a specimen in water in a finger-bowl, with a black tile for the background, find the following from the sub-umbrellar surface:
The shape of the animal. Is the margin perfectly circular or regularly indented? Are all of the marginal portions similar?
Four large, fringed oral arms or lips hang from the corners of the nearly square mouth, which is located in the center. Notice how each arm is similar to a long, narrow leaf, with the sides folded especially along their margins. Examine the arms for nematocysts. Do you understand how the animal gets its food? If the arm edges appear to be covered with dark specks and granules, examine to see if embryos may not be entangled.
The mouth is found to lead by a short gullet into a rather spacious stomach, which is produced in the region between each two corners of the mouth to form a gastric pouch. Determine the shape of the stomach.
The remaining parts of the digestive (and also in this case circulatory) system include the numerous radial canals and the single circumferential canal.
Directly beneath each oral arm a per-radial canal is given off, which, at a short distance from the stomach, gives off a branch on either side. The per-radial canal proper usually continues straight to the marginal circumferential canal, without further subdivision, but the two side branches above mentioned in turn subdivide several times.
From the peripheral wall of each gastric pouch three canals pass toward the margin; the middle one (inter-radial canal) branches somewhat after the manner of the per-radial canals, but the other two (ad-radial canals) continue to the circular canal without further branching
The position of the gastric pouches is made clearly mani¬fest by the gonads, which lie on the floor of the pouches, as frill like structures, horseshoe-shaped, with their open sides toward the mouth The ova or spermatozoa are shed into the stomach and pass out of the mouth Embryos in various stages of development may frequently be found adhering to the oral arms The sexes arc separate. On the sub-umbrellar surface, opposite each gonad, is a little pocket, the sub-genital pit, which opens freely to the outside. Whatever purpose this may serve, it does not function to conduct the genital products to the outside.
Parallel with the inner or concave border of each gonad is a row of delicate gastric filaments. These are supplied with stinging cells, and they may aid in killing live food taken into the stomach These structures are not present in the Hydrozoan medusa.
At the marginal extremity of each per-radial and inter-radial canal there is an incision on the edge of the animal, in which there are sensory organs. In each incision find:
A tentaculocyst in the form of a short, club-like structure containing a prolongation of the circular canal. At its outer extremity are calcareous concretions or lithiles, and a pigment-spot or ocellus. Each tentaculocyst is protected aborally by a hood-like projection, and on the sides by marginal lappets.
Two depressions, one above and the other below the tentaculocyst. These have been assigned olfactory functions, and are called the olfactory pits
Make a drawing showing the profile of the entire animal, and show the structure of at least one quadrant, as seen from the oral surface.
If time permits study a developmental stage, "ephyra," and compare it with the adult.
By way of comparison, examine demonstrations of Cyanea, Dactylometra, Lucernarm, or other forms belonging to this group.
Specimens are quite common on piles, as well as on rocky bottoms, and may be easily observed by means of a glass-bottomed pail. Most of the observations can be made much better on specimens in aquaria, but it is desirable to see their natural surroundings.
Notice the shape and attachment of expanded, living specimens in an aquarium, or in a deep finger-bowl The free end called the disk or peristome, is fringed with tentacles, and the elongated mouth is located in the middle of this area At one or both angles of the mouth the lips are thickened into what is called a siphonoglyph.
Make a drawing of the animal.
Feed a specimen with bits of mashed clam to ascertain its manner of taking in food. Drop bits on the tentacles at one time, and disk at another.
Endeavor also to determine whether there are currents constantly passing in or out of the outh that are due to ciliary action
Irritate the animal and observe its manner of contraction. When fully contracted, if the irritation is continued, threadlike structures, acontia, are thrust out through minute pores, cinclidcs, in the body-wall.
Make a drawing of the contracted animal
Internal Anatomy — Using preserved material, place the edge of a razor across the peristomial area, at right angles to the mouth-slit, and divide the animal from disk to base into halves.
- Note the extent of the esophagus and siphonoglyphcs; they lead into the calenteric chamber. Find the extent of this chamber, and the method of its subdivision by delicate partitions, the mesenteries, or septa Are all of the mesenteries alike?
- Forming the free edges of the mesenteries, below the esophagus, arc the convoluted mesenteric filaments, which are secretory organs that are probably equivalent to the gastric filaments of the Scyphozoa.
- Quite near the bases of the mesenteries are the attachments of the acontia. What relation have they to the mesenteric filaments?
- Also located on the mesenteries, and arranged parallel to the filaments, but back from the edge a bit, are the reproductive organs or gonads. Are they found on all of the mesenteries' The ova or spermatozoa are shed into the coelenteric chamber and pass out through the mouth
Cut one of the halves of your specimen transversely in the region of the esophagus, and study the arrangements of the mesenteries, their attachments, etc.
How many pairs of primary mesenteries, i. e , those attached both to the outer body-wall and to the esophagus, are there? The directive septa are those at the angles of the esophageal tube. The portion of the coelenteric cavity between any two pairs of mesenteries is termed an inter-radial chamber. The space between the two mesenteries of each pair is called an intra-radial chamber.
Carefully determine the disposition of the longitudinal retractor muscles on the mesenteries. Do they occupy similar positions on all of the mesenteries?
Examine the upper parts of the mesenteries for openings, septal stomata, that put the chambers in communication
Are the tentacles solid or hollow?
Make a drawing of a longitudinal section and another of a cross-section Put into these all of the points of the anatomy you have seen.