186 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



tige of ganglia, but to descend at once to the 

 lowest forms of animal and vegetable life, rhythmic 

 action may here be said to be the rule rather than 

 the exception. The beautifully regular motions 

 observable in some Alg?e, Diatomacese, and Ocilla- 

 torise, in countless numbers of Infusoria, Anthero- 

 zoids, and Spermatozoa, in ciliary action, and even 

 in the petioles of Hedysarum gyrano, are all in- 

 stances (to which many others might be added) of 

 rhythmical action where the presence of ganglia 

 is out of the question. Again, in a general way, 

 is it not just as we recede from these primitive 

 forms of contractile tissue that we find rhythmic 

 action to become less usual ? And, if this is so, 

 may it not be that those contractile tissues which 

 in the higher animals manifest rhythmic action are 

 the contractile tissues which have longest retained 

 their primitive endowment of rhythmicality ? To 

 my mind it seems hard to decide in what respect 

 the beating of a Snail's heart differs from that of 

 the pulsatile vesicles of the Infusoria; and I do 

 not think it would be much easier to decide in what 

 essential respect it differs from the beating of the 

 Mammalian heart. The mere fact that the presence 

 of ganglia can be proved in the one case and not 

 in the other, seems to me scarcely to justify the 

 conclusion that the rhythm is in the one case 

 wholly dependent, and in the other as wholly in- 

 dependent, of the ganglia. At any rate, this fact, 

 if it is a fact, is not of so self-evident a character 

 as to recommend to us the current theory of gang- 

 lionic action on d priori grounds. 



