310 PHYSIOLOGY OF THE INVERTEBRATA. 



the influence of the poison has been removed, even though 

 this has acted to the extent of totally suspending irritability. 

 In other words, there is no poison in the above list, which has 

 the property, when applied to the Medusce, of destroying life 

 till long after it has destroyed all signs of irritability." As 

 an explanation of this peculiarity it should be borne in mind 

 " that in the Medusa there are no nervous centres of such 

 vital importance to the organism that any temporary sus- 

 pension of their functions is followed by immediate death. 

 Therefore, in these animals, the various central nerve-poisons 

 are at liberty, so to speak, to exert their full influence on all 

 the excitable tissues without having the course of their action 

 interrupted by premature death of the organism, which in 

 higher animals necessarily follows the early attack of the 

 poison on a vital nerve-centre." Then, again, the mode of 

 administering the poisons to the Medusce was different from 

 that which is generally used when administering them to the 

 higher animals. 



(15) Romanes' researches prove that the phenomena of 

 muscular tonus, as they occur in Sarsia, tend more in favour 

 of the exhaustion, than of the resistance, theory of ganglionic 

 action. " The exhaustion theory supposes that the rhythm is 

 largely due to the periodic process of exhaustion and recovery 

 on the part of the responding tissues." 



Besides the researches on the nervous systems of the 

 Medusce, Dr. Eimer * has investigated the nervous system of 

 the Gtenopliora. In these animals the mesoderm contains 

 numberless fibrils, varying in diameter from ^x^tt^ to xtsutt 

 of an inch. " These fibrils present numerous minute varico- 

 sities, and, at intervals, larger swellings which contain nuclei, 

 each with a large and refracting nucleolus. These fibrils take 

 a straight course, branch dichotomously, and end in still finer 

 filaments, which also divide, but become no smaller. They 

 terminate partly in ganglionic cells, partly in muscular fibres, 

 partly in the cells of the ectoderm and endoderm. Many of 

 * Zoologische Studien auf Capri, 1873. 



