The Polymorphic Transformations of Obelia
Authors: Berrilli N. J. 
Published: April 2021

The growth cycle of O. commissularis McCrady is analysed in terms of tissue mass and shape, cell number and cell transformations.

A detailed analysis is made of stolonic growth typical of both free and attached stolons. Cells generally pass in succession through a phase of rapid non-vacuolate division, slower vacuolate division, a vacuolate recovery phase, a pulsation phase, and a resting phase. Cells in the resting phase may recommence the cycle at any time.

Chitin is almost certainly secreted by specialized glandular cells laden with highly refringent granules. Chitin is at first secreted as a viscous film, but rapid polymerization makes initial curvatures permanent features, such as annular rings. Local growth of epidermis in some way dissolves thick polymerized chitin.

All branches growing from a free or attached stolon and not forming at pre-existing junctions, undergo a phase of primary annular growth, a simple stolonic phase, and a second annular phase. Annulations are formed as the result of the epidermis rhythmically forming a series of terminal blobs in advance of the growing tip of the endodermis which penetrates them successively but belatedly. In the first series the blobs decrease in size until both tissue layers grow evenly as stolonic growth, the second series increase in size until a critical threshold size is attained which represents the hydranth rudiment.

The hydranth rudiment grows as a unit. Cells for the most part divide at the distal end to form a disk of increasing diameter. Cells are progressively added from the disk margin to the wall of the cylinder or body, where they pass through the pulsation phase and pull away from the surrounding perisarc. There is no further growth of the wall itself. The anterior disk becomes polygonal in outline, and the number of sides is always equal to half the number of tentacles to be formed, the tentacle number varying from 16 to 24.

Gonangia develop only from stem junctions already formed. Growth is annulated as in the case of the hydranth, but the epidermal blobs increase in size from the first and the final blob which represents the gonangium rudiment proper is somewhat larger than the comparable hydranth rudiment and is also relatively wider in the transverse plane.

The gonangium develops in the same manner as the hydranth up to the stage at which a hydranth would be about to form tentacle rudiments. At this time in gonangial development, the formation of medusa buds from the anterior body-wall immediately below the tentacle disk is so demanding that either one or both tissue layers of the anterior disk begin to shrink. The gonangium is interpreted as a relatively large hydranth whose later development is aborted by the precocious onset of medusa-bud formation.

Medusa buds arise from the gonangial wall while it itself is in progress of growth and arise in series as annular groups commencing anteriorly as the gonangial wall progressively attains an adequate girth.

The medusa bud arises directly from the wall and not from the terminal of a growing stolon as in hydranth and gonangium. The cells are small, indicating relative rapidity of division, and an entocodon, essential for medusoid organization, is formed by apical thickening of the medusa-bud epidermis, followed by segregation.

As medusae develop, the blastostyle to which they are attached grows and elongates towards the anterior end, thereby carrying medusae in effect through the distal aperture of the ripe gonangium, in order of their seniority. Upon liberation the medusae all have sixteen tentacles and no sign of gonads.