ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 311 



suddenly springs into rapid rhythmical pulsation, regular and sustained 

 like clockwork, and continuing indefinitely in normal sea-water without 

 further external stimulation. The waves of pulsation all arise from 

 the stimulated point, and the labyrinth of sub-umbrella tissue around 

 the centre must form a closed circuit. When each wave returns to the 

 centre it is reinforced and again sent out through the circuit. The 

 centre sustains the pulsation, and once established it remains at a fixed 

 point, while the disk continues to pulsate. Sustained ])ulsation in disks 

 (without marginal sense-organs) occurs only in tissue forming a complete 

 circuit, and depends upon an electric transmission of energy. The 

 pulsation is fully twice as fast as that of a normal Medusa, its rate de- 

 pending on the length of the circuit, and it is self-sustaining (i.e. 

 sustained by internal stimuli) once it is started by an external momen- 

 tary stimulus. The stimulus which causes pulsation is transmitted by 

 the diffuse nervous or epithelial elements of the sub-umbrella. 



The paralysed disk is stimulated by chlorides of Xa, K, Ca, etc.,. 

 especially by the sodium chloride of the sea-water, but magnesium salts 

 control and inhibit this stimulation. The same is true in regard to the 

 heart of Salpa, the heart of the embryo loggerhead turtle, and the 

 barnacle's cirri. The NaCl, K, and Ca of the sea-water unite in stimu- 

 lating pulsation of the Cassiopea disk and in resisting the stupefying 

 effect of the Mg. All four salts conjointly produce, in sea-water, an 

 indifferent, or balanced, fluid, which neither stimulates nor stupefies the 

 disk of Cassiopea, and permits a recurring internal stimulus to produce 

 rhythmic movement. 



The chief results of Mayer's research are the discovery of a new 

 method of restoring pulsation in paralysed Medusse, and also that, 

 magnesium plays a most important role in restraining, controlling, and 

 prolonging pulsation in animal organisms. In Cassiopea the ectodermal, 

 epithelial, or diffuse nervous elements of the sub-umbrella transmit the 

 stimulus which produces rhythmical contraction. 



Rhythmical pulsation can be maintained only when a stimulus and an 

 inhibitor counteract one another, and cause the organism to be upon the 

 threshold of stimulation, thus permitting weak internal stimuli to pro-^ 

 mote periodic contractions. 



Supposed Australasian Hydroid in North Sea.* — James Ritchie 

 reports the occurrence in the North Sea of Sertularia elongata Lamx, 

 which is limited to Australasian waters. It was attached to a twig, 

 apparently belonging to one of the Cymodocete (Potamogetonacese), 

 which, with one exception (the north temperate Phucagrostis major), 

 occur in tropical or south temperate seas. As the twig was not Phuca- 

 grostis major it seems likely that it has drifted with the attached 

 hydroid to the North Sea probably from an Australian locality. The 

 specimen, which was a fine one, was found on the net of a trawler. ■ 



Intranuclear Crystals in Tubularia.f — Jovan Hadzi finds that- 

 the nuclei of the ectoderm cells of the aboral tentacles of Tuhularia 

 mesembrganthemiim often show a peculiar crystalloid plate of hexagonal 

 form. It is probably a symptom of some degenerative change. 



* Proc. Roy. Phys. Soc. Edinburgh, xvii. (1907) pp. 78-81 (1 pi.). 

 t Zool. Auzeig., xxxi. (1907) pp. 375-9 (7 figs.). 



