Echinoderm Larvce with Two Water- Vascular Systems. 323 



direct improvement of conductivity or (h) secondarily through the slowing 

 of the rate of succession which they may induce, i.e. by lowering excitability, 

 provided that this effect is not attended by a proportionate lowering of 

 conductivity. Adrenalin is notably useful in this respect, as indicated by 

 the remarkable improvement in conduction often seen under its influence, 

 especially evident in the auricles, where a strikingly slow contraction wave, 

 present during gravely depressed conduction, may be replaced by an 

 approximation or a return to the normal type. Hence the special utility of 

 adrenalin in dealing with forms of slow coarse fibrillation, already described, 

 and also with fibrillar beats — unless the damage in the latter case has been 

 carried to an irreparable stage (fig. 9). 



The success of the above-mentioned methods of obtaining recovery from- 

 typical fibrillation, induced by means that did not permanently damage the 

 heart, has been such that in recent years of experimentation there has not 

 been failure in any instance. 



For valued assistance in some of the experiments of this investigation, I 

 have to record my thanks to Drs. G. Spencer Melvin and J. E. Murray. 

 A portion of the costs was defrayed by a grant from the Carnegie Trust. 



The Artificial Production of Echinoderm Larvce. tvith Two Water- 

 Vascular Systems, and also of Larvce Devoid of a Water- 

 Vascular System. 



By E. W. MacBride, T.E.S. 



(Received January 18, 1918.) 



[Plates 4-10.] 



The development of Echinoderms has been characterised, and with 

 justice, as the most remarkable ontogenetic change in the animal kingdom. 

 For the larva is an almost perfect example of a simple, bilaterally 

 symmetrical Metazoon, and the amazing thing is, not that the radially 

 symmetrical adult should develop out of a bilaterally symmetrical larva, 

 but that the axis of symmetry of the radial adult should cut the principal 

 axis of the bilateral larva at an angle which approaches 90°. 



In the three orders Asteroidea, Ophiuroidea, and Echinoidea, the general 

 ?0L. xc. — B. , 2 D 



