DEPARTMENT OF MARINE BIOLOGY. 187 



while the other touches it Hghtly. The wave also travels from rested 

 toward tired or from, fresh toward exhausted tissue. 



Magnesium is not an active inhibitor of nerve conduction in Cassi- 

 opea, but is nearly if not quite neutral, having little or no more effect 

 than distilled water or a non-electrolyte, such as dextrose. 



Sodium and potassium behave alike in respect to nerve conduction, 

 but potassium is much more active than sodium, as one would expect 

 from its higher atomic weight. 



Calcium combines to form a sodium-calcium proteid which tends to 

 produce muscular tetanus, but this tendency is offset by the effect of 

 magnesium. Calcium can not produce tetanus unless sodium be present. 



The rate of nerve conduction in Cassiopea is more rapid in 95 per 

 cent sea- water than in natural sea-water. It is again about normal in 

 80 per cent sea-water, below which it falls off steadily. 



Tropical marine animals are more injuriously affected by slight 

 changes of temperature above or below the normal than are those of 

 colder seas, and (as Harvey showed) the tropical forms live within a 

 few degrees of their upper death temperature, and can withstand cool- 

 ing much better than they can heating. It seems probable that the 

 effect of high temperature is to produce asphyxiation, the oxygen of the 

 sea-water being insufficient to maintain the increased metabolism of the 

 animal. 



A study of the coral reefs of the Murray Islands, Torres Straits, 

 shows that temperature is the most important factor determining the 

 habitats of the various sorts of corals over the reef flats, for, in common 

 with other tropical marine animals, corals live within less than 10° C. 

 of their death temperature. High temperature probably produces 

 death by causing asphyxiation, hence those corals which are most 

 easily affected by high temperatures are usually also those which are 

 most easily smothered by mud and must, therefore, live in pure ocean- 

 water free from silt, and can not survive in the hot, muddy water near 

 shore. The physiological reactions of Australian corals are essentially 

 similar to those of Florida, and natural selection has not improved 

 them. For example, the Torres Straits corals withstand low tempera- 

 ture as well as do those of Florida, and the Florida corals are quite as 

 well able to resist heat as are those of Torres Straits. 



It appears that the rate of starvation in Cassiopea can be expressed 

 by the formula y = W {l — aY, where y is the weight at the end of x 

 days, W is the original weight, and a is a constant less than unit3\ This 

 formula shows that the substances which sustain the starving animal 

 remain the same throughout the period of starvation. 



Professor Henry S. Pratt found a new species of trematode at Tortu- 

 gas, Monocotyle floridana, in which the ahmentary tract, at least in 

 some individuals, is complete. 



Dr. Edwin E. Reinke has made an interesting cytological and physio- 

 logical studj'^ of the dimorphic male sex-cells of various prosobranch 



