RHIZOSTOMiB — UHIZOSTOMA. 7()1 



This medusa is common in the Mediterranean. It is touiul tluoughout the year, but is 

 most abundant from June until August, becoming ripe in August and September. Very small 

 medusae are often tound in June. Mature individuals are occasionally seen in midwinter. 



Claus has studied the development of the pelagic eph)ra. When 3.5 mm. in diameter 

 the cphyra has a central, cruciform mouth, the 4 lips of which are lined by a row of knobbed 

 tentacles as in Aurdlia. There are 8 pairs (16) of velar lappets and 8 pairs (16) rhopalar lap- 

 pets. 16 radial-canals and a simple, circular canal. In this stage the 8 adradial canals end 

 in the ring-canal, but the 8 others go to the bell-margin. The velar lappets develop in pairs, 

 as in the Discomedusae and in Stomolophtis, not singly as in Aurellia or Cotylorhiza. The 8 

 mouth-arms arise from paired, terminal folds of the 4 primary rays of the cruciform, cen- 

 tral mouth. A detailed description of the young ephyra is given by Claus, 1885. 



The rhythmical pulsation of this medusa has been studied by von Uexkiill, who found 

 that it the marginal sense-organs be mechanically confined the pulsation is hindered. His 

 conclusion, however, that the stimulus which produces pulsation is mechanical in nature and 

 may be likened to that produced by the clapper of a bell in striking against the margin seems 

 to me improbable. Pressure upon the nerve-center might readily interfere with the activity 

 of the sense-club, and any confinement which cuts off the supply of soluble calcium from the 

 sea-water would soon cause pulsation to cease. 



A very suggestive and important series of studies of the nature of the pulsation stimulus 

 in this medusa and in Cotvlorhiza tuhcrculata was carried out by Bethe, 1903-1909. He finds, 

 in 1903, that under normal conditions hundreds of pulsations follow quite regularly one after 

 another, with only an occasional pause of brief duration. The medusa pulsates almost inces- 

 santly. According to Bethe there are many analogies between the pulsation of this medusa 

 and that of the vertebrate heart. For example, the "all or none" principle applies to 

 medusae, as does also the phenomena of the extra systole and corresponding compensation- 

 period of rest. The medusae also show a refractory stage during systole in w'hich they are 

 insensible to stimuli, as was demonstrated by Marey, 1876 (Travaux du lab., p. 73), for the 

 vertebrate heart. 



There is an increase in the time that elapses between stimulation and response, and also 

 in the duration of the pulsation itself as the temperature is lowered from 25° to 13° C. The 

 pulsation-stimulus is nervous in nature, being transmitted by the diffuse, nervous network 

 of the subumbrella. Indeed, there are areas of the subumbrella which are wholly without 

 muscles; nevertheless the pulsation-stimulus passes freely over these to the muscular areas 

 beyond. Bethe gives a good series of figures showing the histological character of the nerve 

 plexus which forms a network between the epithelium and the deep-h'ing, muscular layer of 

 the subumbrella. The sense-organs are physiologically speaking only highly differentiated 

 parts of the nerve-plexus of the subumbrella. 



Under normal conditions the pulsation-stimulus originates in the marginal sense-organs, 

 yet in medusae (such as Cotylorhiza) which have an inner zone of radial and an outer zone of 

 circular muscles in the subumbrella the radial-muscles contract bcjorc the circular, although 

 the\- are farther away from the sense-organs. This is due, as Bethe shows, to the fact that the 

 latent period (/. e., the time that elapses between stimulus and response) is longer for the cir- 

 cular than for the radial muscles. 



The pulsation is a reflex due to a constantly present stimulus, and the refractory stage 

 produces periodicity (rhythm) in the responses. The nerves can not send forth a new con- 

 traction-stimulus until a definite period of rest has elapsed. 



Bethe, 1908, 09 (Pfliiger's Archiv. fur ges. Physiologie, Bdn. 124 and 127), has continued 

 his studies of the rhythmical pulsation of Rhizosfonin piilmo at Naples. He finds that artificial 

 sea-water will not sustain life and pulsation as well as does natural sea-water unless a small 

 amount of CaCog be added to the solution. This improvement of the artificial sea-water is not 

 due he believes to the addition of Ca or to the neutralization of an acid, but is caused by the 

 presence of the undissociated molecules of CaCo,. It will be recalled that Rogers, 1905 

 (Journal Experimental Zool., vol. 2, p. 249), found that the addition of small amounts of CaCo., 

 to solutions containing the pulsating heart of the crab Brnchyiujtus had a beneficial effect. He 

 attributed this, however, to the neutralization of free acid in the solutions. 



