High-speed microcinematography of nematocyst discharge in Hydra attenuata Pall. shows that this specialized exocytosis, which appears to result from an increase in the intracapsular pressure, requires a total of about 3 milliseconds. A maximum velocity of 2 meters per second is generated, corresponding to an acceleration of 40,000g. Thus nematocyst discharge is one of the fastest cellular processes in nature.

The members of the phylum Cnidaria (corals, sea anemones, medusae) are all equipped with stinging cells (cnidocytes, nematocytes), which serve mainly in prey capture and defense. The secretory product of these cells is a most complicated extrusome consisting of a cyst containing a tubule and a liquid matrix. Mechanical stimulation of the cell's cnidocil apparatus by a prey or an offender leads via bioelectrical signal transduction to the explosive discharge of the cnidocyst. In stenoteles of Hydra this process, during which the tubule is everted out of the cyst, takes less than 3 msec. The forces involved are partly due to spring‐like tensions stored in the collagenous structural compartment, and partly to an osmotically generated intracapsular pressure, which can amount to 150 bar (1.5 × 107 Pa). The osmotic machinery depends on the presence in the cyst's matrix of inorganic cations (either K+, Mg2+ or Ca2+) and rare polyanions (poly‐γ‐L‐glutamates), which, so far, have not been reported from recently evolved eukaryotes. The discharging cyst acts like a self‐reloading syringe, injecting poison and other components into the target. Since the cnidocytes are incapable of regenerating their exocytosed cysts, they have to be replaced by new cells derived by differentiation from pluripotent stem cells (interstitial cells).

Summary 1. This article reviews the present state of knowledge on regeneration in the Hydrozoa and some of the problems related to it. The available data derived from descriptive and experimental studies are confined to only a few genera (Hydra, Tubularia, Cordylophora, Corymorpha, Campanularia, Pennaria). Without enumerating all possible types of regeneration, a number of the most significant cases are examined from various points of view. 2. Some of the anatomical and physiological particularities exhibited by the Hydrozoa have been emphasized as a possible basis for their remarkable morpho‐genetic abilities. The diploblastic architecture of a hydroid polyp is of a relatively simple design, lacking–in contrast to the higher Metazoa–a number of structurally and functionally well‐circumscribed organs. AH vital functions are distributed amongst a very small number of cell types, which have during their differentiation retained a relatively high standard of physiological autonomy and adaptability, being able to undergo, within certain limits, structural and functional transformation. Such differentiated somatic cells are subjected to normal senescence, which is compensated by a continuous (Hydra) or periodical (Tubularia) replacement activity. This rejuvenation process depends upon the presence of a permanent self‐proliferating stock of indifferent replacement cells (interstitial cells). 3. In a single polyp or in a colony the quantitative pattern of the morphogenetic or regenerative potentials is inversely proportional not only to the degree of tissue differentiation (Needham, 1952), but also to the progress of physiological ageing. In this particular case the age of a tissue or body fraction is not determined so much by the age of the somatic elements as by the number of available undifferentiated replacement‐cells. In the hydrocaulus of Tubularia the graded morphogenetic potentials are proportional to the degree of basipetal ageing of the coenosarc tissue. 4. The organogenetic pattern of a developing regenerate is constituted by differentiated somatic cells, which migrate to or are shifted towards the site of recon‐stitution together with the undifferentiated interstitial cells. The function of organizer is delegated to the endoderm layer, while the interstitial cells provide the regenerate with additional young elements which differentiate according to their position in the pre‐established organogenetic pattern. The quantitative behaviour of nucleic acids, free amino acids and proteolytic enzymes suggests that the structural differentiation of the regenerate is accompanied by a synthesis of new, probably organ‐specific proteins. 5. The mechanism by which the regenerating system controls the morphogenetic activities includes an inhibitory principle. In fact the hydranths of various species were found to produce a factor which has the faculty of reversibly inhibiting the recon‐stitution of an equivalent structure in the same morphogenetically active system. Within a developing regenerate the rate of production of...

This light- and electron-microscopic study has investigated the structure, the morphodynamics of discharge, and the impact of the stenotele cyst of Hydra attenuata (Hydrozoa, Cnidaria) on the prey's integument. The triggered capsule, which is ejected from the cell, discharges its tubular content (shaft, stylets and tubule) by a process of evagination. In doing so the three joined stylets punch a hole into the cuticle of the prey through which the long evaginating tubule penetrates into the interior of the target. The behaviour of the tubule is described in detail and the functional significances of the various parts of the capsule are discussed.

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