4 Papers from the Marine Biological Laboratory at Tortugas. 



can not contract an electrical stimulus does not stop the cilia. The 

 stopping of the cilia is therefore dependent upon the contraction of the 

 muscles. It appears then that the stimulus which produces ciliary 

 movement tends to cause muscular relaxation but is too weak to prevent 

 muscular movement, but the stimulus which produces muscular contrac- 

 tion is of an opposite nature, and is more energetic than that required to 

 maintain ciliary movement and completely overpowers it, stopping the 

 cilia when the muscles contract. Considering all things which normally 

 affect the animal, whatever stimulates the neuro-muscular system inhibits 

 ciliary movement, and whatever stimulates cilia depresses neuro-muscular 

 activity. 



GENERAL CONCLUSIONS. 



A research published in 1908 ^ leads me to believe that in Scypho- 

 medusae each pulsation is due to a stimulus produced by the constant 

 setting free of ionic sodium in the marginal sense-clubs. Sodium oxalate 

 appears to be constantly forming in the entoderm of the sense-club. 

 This precipitates the calcium which constantly enters the sense-club 

 from the surrounding sea-water, and thus insoluble crystals of CaC204 

 are formed while NaCl is set free. The Na ion is a powerful nervous 

 stimulant, and being maintained thus in slight excess in the sense-clubs 

 it acts as a constantly present minimal stimulus which produces periodic 

 responses. It will be recalled that Romanes found that constantly 

 present minimal electrical stimuli could cause periodically recurring 

 pulsation in Scyphomedusae. 



Pulsation can not be maintained by the sense-organs unless calcium 

 constantly enters them to form the crystals of calcium oxalate and to 

 set free the ionic sodium, and all movement soon ceases in sea-water 

 deprived of calcium. 



In Cephalopods, Veligers, marine Annelids, Barnacles (JLepas), and 

 Ctenophorae, the sodium of the sea-water is a powerful neuro-muscular 

 stimulant, whereas the magnesium, calcium, and potassium are inhibitors 

 and counterbalance the stimulating effect of the sodium, giving a neutral 

 or, more properly speaking, balanced fluid, thus permitting weak inter- 

 nally engendered stimuli to produce movements. This is very evident in 

 the case of animals placed in distilled water, wherein they may move 

 normally, although no direct stimulus to produce movement can come 

 from the surrounding medium. Yet fresh-water animals react to sodium, 

 magnesium, potassium, calcium, NH^Cl, and CO2 as do marine animals. 

 Physiological conditions in vertebrates are often different from those in 

 invertebrates, but it will be recalled that Martin, 1906,^ concludes that 

 the vertebrate heart is maintained in rhythmical activity by an inner 

 stimulus due to the metabolic activity of the heart's own tissue, and 

 its pulsation is not caused by an external stimulus due to the ions of 

 the inorganic salts of the blood. Howell has also come to the conclusion 

 that an inner and not an external stimulus causes the heart's activity, 



» Papers from the Tortugas Laboratory, vol. i, pp. 115-131; Publication 

 No. 102, Carnegie Institution of Washington. 



* Martin, 1906, American Journal of Physiol., vol 16, pp. 191-220. 



