Table of Contents
( HYDROZOA )
The colonies form dense tufts of long, tangled, sparsely branched stems, three to four inches high. The hydranths have two rows of short tentacles: a row about the mouth and a proximal row of 16-25 shorter tentacles. See the diagram by Hyman (1940). The colonies grow profusely on piles and docks in the Woods Hole, Mass., area.
A. Care of Adults: Select a good-sized colony, examine it with a hand lens or dissecting microscope, and clip off those branches containing the best embryological material. The hydroids should not be crowded, and the sea water should be changed frequently. They are very sensitive to increases in temperature.
B. Methods of Observation: Motile sperm can be obtained by crushing the gonophores on a slide; eggs and larvae can be teased from the female gonophores with fine needles. Older larvae, close to the time of hatching, are visible in situ. Actinulae escaping from the parent colony will not develop without special feeding. If mature colonies are allowed to stand for several hours in a large dish, the actinulae will be shed first, followed by earlier and earlier developmental stages, until finally cleavage stages are obtained.
A. Asexual Reproduction: The gonosomes of a mature specimen are long, branched stalks which grow in a dense cluster between the two circles of tentacles. The gonophores are budded off along the length of the gonosomes; they are quite markedly reduced medusae which never become free-swimming, and usually show no signs of radial or circular canals. The male and female gonophores are produced by separate colonies. The male gonophores are balloon-like structures lacking tentacles; within them, a cloudy mass of sperm can be seen surrounding the dark red spadix, which is actually the manubrium of the medusoid. The female gonophores, containing the eggs and developing larvae, usually have four blunt, knob-like tentacles at the distal end, although occasionally one or more of these is slightly elongated. Details of gonophore development can be found in the paper by Goette ( 1907 ) .
B. Sexual Reproduction: Although many oocytes are present in the young gonophores, only a few reach maturity. These favored oocytes engulf and absorb the "nurse" eggs, which are arrested in the primary oocyte stage. The sperm probably enters the egg before the polar bodies are given off, although there is some controversy on this point. When ripe, the egg is very large approximately 400 microns) and somewhat irregular in shape (Allen, 1900; Hargitt, 1909, 1919). Fertilization and early development take place within the modified medusa. Cleavage is often irregular and apparently either a coeloblastula or a solid morula can be formed (Lowe, 1926; Hargitt, 1909). Gastrulation of the coeloblastula is described as a mixture of delamination and multipolar proliferation (Benoit, 1925; Hargitt, 1909). The embryo is thus converted into an oval, solid mass of cells which eventually flattens to a disc. Irregular coalescing spaces appear in the endoderm, marking the beginning of the adult coelenteron. Blunt protuberances at the edge of the disc are rudiments of the aboral tentacles of the adult; these rapidly elongate and bend toward the future aboral end of the body. C. Later Stages of Development and Metamorphosis: The entire embryo elongates in the direction of the oral-aboral axis and becomes cylindrical. The oral end is perforated by the mouth and a series of small protuberances, the oral tentacles, develop about this opening. When this stage is reached, the "actinula," which may be considered a precociously metamorphosing form, part planula and part polyp, leaves the gonophore. It then creeps about on the substrate by means of the aboral tentacles; although the mouth is carried downward at this time, the attachment is made by the aboral pole. The attached polyp increases rapidly in height, and buds off daughter hydroids along its sides. The creeping stolons, which bud off secondary upright shoots, are developed from the base of the primary polyp. Details of actinula formation are described by MacBride (1914).
ALLEN, C. M., 1900. A contribution to the development of Parypha crocea. Biol. Bull., 1: 291-315.
BENOIT, P., 1925. L'ovogenese et les premiers stades du developpement chez la Myriothele et chez la Tubulaire. Arch. de Zool. Exp., 64: 85-326.
BERRILL, N. J., 1952. Growth and form in gymnoblastic hydroids. V. Growth cycle in Tubularia. J. Morph., 90: 583-601.
GOETTE, A., 1907. Vergleichende Entwicklungsgeschichte der Geschlechtsindividuen der Hydropolypen. Zeitschr. f. wiss. Zool., 87: 1-336.
HARGITT, G. T., 1909. Maturation, fertilization, and segmentation of Pennaria tiarella (Ayres) and of Tubularia crocea (Ag.). Bull. Mus. Comp. Zool., Harvard, 53: 162-212.
HARGITT, G. T., 1919. Germ cells of coelenterates. VI. General considerations, discussion conclusions. J. Morph., 33: 1-58.
HYMAN, L. H., 1940. The Invertebrates: Protozoa through Ctenophora. McGraw-Hill Book Co., New York.
LOWE, E., 1926. The embryology of Tubularia larynx (Allm.). Quart. J. Micr.. Sci., 70: 599-627
MAcBRIDE, E. W., 1914. Text-Book of Embryology. Vol. I. Invertebrata. Macmillan and Co., Ltd., London.