ANTHOZOA HYDROIDA. 13 



The circulating fluid is loaded with minute granules of 

 nearly equal volume and shape, and endowed with a proper 

 mobility. It moves within the tubular divisions of the poly- 

 pidom, ascending on one side of the tube and descending on 

 the other, or ascending and descending in a canal in the cen- 

 tre of the living pulp. It has no central point to start from, 

 and no fixed goal to reach before it commences its return ; nei- 

 ther is it always steady nor constant in its course, for the stream 

 may be seen to vary in velocity, to stop at uncertain inter- 

 vals, to retrograde on the line it had advanced upon, and to 

 exhibit other partial irregularities without any obvious cause. 

 In its usual course the fluid flows on uninterruptedly up the 

 stem until it meets with a knot or branch, when it is thrown 

 into a slight eddy, or divides itself to follow up the ramifica- 

 tions of the polypidom ; and when the limit has been reached, 

 either just below the base of the polype, or in the very sto- 

 mach of the creature, the granules turn round and pass over to 

 the other side to run their reversed course. " If the currents be 

 designedly obstructed in any part of the stem, those in the 

 branches go on without interruption, and independently of 

 the rest. The most remarkable circumstance attending these 

 streams of fluid is, that they appear to traverse the cavity of 

 the stomach itself, flowing from the axis of the stem into that 

 organ, and returning into the stem, without any visible cause 

 determining these movements." * 



The power which sets in motion and maintains this cur- 

 rent is yet undiscovered. Spallanzani suggested that it might 

 be owing to the elasticity of the sides of the cell and of the 

 stalk acting upon a fluid by which they had been for a time 

 over-distended ; " but this hypothesis," he himself adds, " is 

 not only unsupported by proofs, but insufficient to explain the 

 phenomena."^ The fact is, that the parietes of the tube and 

 of the cells are not elastic in that direction. Professor Grant 

 asserts that it depends on the action of minute vibratile cilia, 

 " the common agents of all analogous movements in the 



Roget's Bridge w. Treat, vol. ii. p. 233. See also Tiedemann's Corap. Physiol. 

 p. 150. Ent. Mag. vol. iii. p. 174. Grant's Outlines of Comp. Anat. p. 429-30. Van 

 Beneden, M6m. sur les Campanulaires, p. 17, 18 ; and his Rech. sur les Tubulaires, 

 p. 20. 



t Travels, iv. p. 292. 



