20 Coelenterata. 



is held down, "by the tentacles, to the disc which at this point begins to sink 

 inwards, the mouth opens, the walls of the stomodaeum protrude as large lobes 

 and the region between the mouth and the food begins to contract. The ten- 

 tacles of this region almost completely disappear while the other parts of the 

 disc and their tentacles become plump and enlarged. The stomodseal lobes 

 increase in size, becoming 2-4 cm. long, extend towards the food, enclose it, 

 and it is pressed into the internal cavity. By these contractions and expan- 

 sions the mouth may be moved from the centre of a disc 10 cm. in diam. to 

 within a cm. of the edge. 20 or more pieces of crab may thus be successively 

 taken, but the later reactions of a series become much slower, the animal seern- 

 iiig to become gradually satiated. Food may be taken by the tentacles and 

 held a long time before being moved to the mouth, it may then be taken, or, 

 on coming into contact with the stomodseal lobes or before, the process stops 

 and the food is rejected. When fully satiated, food may be rejected as soon 

 as it comes into contact with the disc, but after one or more pieces have 

 been rejected another piece may be accepted. The reaction to a given stimulus 

 is not invariable but depends on the state of the internal processes. When 

 several pieces of meat are placed at the same time on different parts of the 

 disc all may be carried to the mouth if the animal is hungry, thus 6 pieces 

 may be swallowed either simultaneously or successively. In some cases when 

 2 pieces of meat are placed on the disc, one is swallowed and the other rejected, 

 but the latter may be taken if placed on the disc after the first piece has 

 been disposed of. Adding new pieces while others are being swallowed often 

 causes interference with this process. The rejecting reaction by which the 

 disc is cleared when rejected food or de'bris fall upon it is carried out in 

 several ways. The tentacles bearing the food or de'bris become thin and slender 

 and lie flat against the disc which in this region begins to stretch. As a 

 result the waste mass is left on a smooth surface fully exposed to the action 

 of waves or currents and the slightest disturbance in the water washes it off. 

 If this does not take place at once, swelling occurs so that the debris comes 

 to lie on a strongly convex surface. If the debris is not now removed other 

 reactions set in : if the mass is nearer one edge of the disc, this edge usually 

 begins to sink while at the same time the tentacles between the edge and the 

 debris efface themselves, thus a smooth sloping surface is produced and the 

 mass slides off the disc; the process may be helped by swelling of the region 

 behind the mass, causing it to lie on a steep slope. One portion of the disc 

 after another may act in this way until all the waste matter has been removed, 

 then the disc resumes its flat or concave surface. If the edge bearing the 

 debris cannot be lowered owing to some external impediment, it is raised and 

 the de'bris rolled towards the centre and finally across the disc. The rejecting 

 reaction is characterised by great flexibility and variability, one series of 

 actions after another being used until one is successful. - In Aiptasia (2 sp.) 

 the general contraction does not appear to be due to the spreading of a con- 

 traction wave from one part of the animal to another but suggests rapid (nervous) 

 conduction. When hungry the animals take both meat and filter-paper, when 

 satiated they take neither. Usually the tendency to take both ceases at the 

 same point, but sometimes the animal ceases to react to the weaker stimulus 

 (filter-paper) while still accepting meat. It is not the mere mechanical fulness 

 of the ccelenteron which determines acceptance or rejection, but some change 

 in the metabolic processes; filling the coelenteron with filter-paper has not the 

 same effect as filling it with meat, for even when the cavity is so full that 

 pieces of paper are repeatedly disgorged new pieces are readily taken. As 



