Gonionemus was formerly very abundant in the Eel Pond at Woods Hole, Mass., but has now practically disappeared from the vicinity; this seems to be associated with the disappearance of eel grass.

The adults measure about 20 mm. in diameter and are about 10 mm. high; they possess four radial canals on which are located the yellow, ribbon-like gonads. The marginal tentacles vary in number from 16 to 80 and have characteristic adhesive pads near their tips. The sex can be ascertained by examining the animals with a dissecting microscope; the ovaries have a granular appearance, while the testes are homogeneous and translucent.

July to the last week of September; the height of the season is from mid-July to mid-August.

PROCURING AND HANDLING MATERIAL

A. Care of Adults: The adults are hardy and will survive in aquaria for several months. Neither larvae nor adults need running sea water, but they seem to survive best in balanced aquaria where they have a constant supply of diatoms for food.

B. Methods of Observation: Adults gathered in the morning will normally shed their eggs between 6 and 8 P.M. on the day of collection. They can be induced to shed in the morning by keeping them in artificial light during the night to prevent shedding, and then placing them in the dark for an hour. If eggs are desired in the afternoon, the adults should be gathered in the morning and then placed in the dark for an hour, beginning at about 3 P.M. A single animal kept in the laboratory will produce eggs nightly for as long as a week, although the number of eggs obtained decreases after the third day.

Since the fertilized eggs adhere to the surfaces with which they come into contact, it is suggested that the dishes in which shedding animals are placed contain microscope slides. These slides, with the attached eggs and larvae, are then easily manipulated for study.

A The Unfertilized Ovum: The eggs are yellowish in color and measure about 70 microns in diameter. When shed, they are covered with a sticky jelly membrane; after fertilization they rapidly sink to the bottom and adhere to the container. Maturation apparently takes place before shedding, since polar bodies are not found on shed eggs.

B. Cleavage and Gastrulation: Cleavage is total and equal. At the 8-cell stage, there is a rotation of some of the blastomeres to form a flat plate of cells. Continued cleavage gives rise to a hollow blastula consisting of a single layer of cells. These cells develop cilia and, while the blastula is still rotating within its membrane, the endoderm is formed by multipolar delamination. With the formation of this inner layer the blastocoele is eliminated.

C. Time Table of Development: Perkins (1902) gives the following schedule. The times are given in minutes after fertilization; no temperature is specified.

D. Later Stages of Development and Metamorphosis: After leaving the membranes, the larvae elongate and the anterior end (aboral pole) broadens. At this time the planulae are bottom-swimmers. The coelenteron begins to form in the posterior portion of the larva by a rearrangement of the endoderm cells, shortly before metamorphosis. At the same time the cell boundaries of the posterior (oral) pole disappear and this region becomes syncytial. At metamorphosis the larva stops swimming, loses its cilia, and attaches by the broad anterior end. A mouth forms at the free end, and is surrounded at first by two tentacles and later by four. The polyps remain solitary but may give off non-ciliated planula-like buds (frustules) which creep along the bottom and develop into hydroids.

JOSEPH, H., 1925. Zur Morphologie und Entwicklungsgeschichte von Haleremita und Gonionemus. Zeitschr. f. wiss. Zool., 125: 374-434.

MURBACH L., 1895. Preliminary note on the life-history of Gonionemus. J. Morph., 11: 493-496

PERKINS, Y.. F., 1902. The development of Gonionema murbachii. Proc. Acad. Nat. Sci., Philadelphia, 54: 750-790

RUGH R., 1929. Egg laying habits of Gonionemus murbachii in relation to light. Biol. Bull., 57: 261-266.