1912.] The Locomotor Function of the Lantern in Echinus. 105 



enlarged, so that some lowering of pressure is established. This lowering of 

 pressure also affects the lantern ccelom, the walls of which in many parts are 

 thin and highly mobile. Accordingly, the fluid within the gills flows back into 

 the lantern ccelom. On the other hand, when the diaphragm rises (i.e. during 

 retraction), the internal pressure is heightened throughout, and the gills 

 are refilled by an outflow of fluid from the lantern coelom. A corresponding 

 tendency to slightly greater fullness of the tube feet and anal plate can also 

 sometimes be observed. Normally the anus is kept closed. Uexkull (see below) 

 noted that during retraction of the lantern in SpJuerrch i/iv.x r/ra nvlo.ri% when the 

 anus happened to be open, a compensatory outflow of fluid from the last part of 

 the intestine might take place, and suggested that in this way the evacuation 

 of faeces might be aided. He also rightly pointed out that adjustment of 

 internal to external pressure can take place by the passage of water through 

 the mouth. But immediate compensatory flux and reflux can hardly be 

 supposed to take place by the mouth when one remembers the usual 

 condition as regards narrowness of the oesophagus, and the manner in which 

 the oesophageal papillae swell out towards the end of retraction and during 

 the first stage of protrusion. This last circumstance, as Uexkull noted, is an 

 indication that the internal pressure is greatest towards the end of retraction. 

 It is quite possible, however, that the process of swallowing may be aided by 

 the suction which protrusion of the lantern is capable of exercising, 

 supposing the cesophageal canal to be open. 



Paralysis of the gill musculature can also be obtained by rinsing the gill 

 gently with an 8 per cent, solution of chloral hydrate in salt water, and then 

 washing thoroughly with clean water after an interval of 10 or 15 seconds. 

 This produces some slight rigidity of the gill wall along with the paralysis, so 

 that for observational purposes the method is not so good as that of exposure 

 to the air, which leaves the gill wall flaccid. It has, however, the advantage 

 of causing cessation of movement in the surrounding spines, etc., if these have 

 also been rinsed with the chloral. 



If one pierces the shell and the buccal membrane by fine holes into which 

 capillary tubes are inserted, one may sometimes observe in these tubes a 

 slight rise and fall of the fluid level accompanying each retraction and 

 protrusion of the lantern. This is best seen in urchins out of water. But it 

 will be found that with fine tubes there is often failure of response to slight 

 variation of pressure. A very sensitive method, which, however, can only be 

 applied out of water, is to bring the capillary tubes into such a position that 

 drops of escaping ccelomic fluid just begin to form at their outer ends. The 

 rate of dropping is hastened during retraction of the lantern and lessened or 

 reduced to zero during protrusion. 



