Mr. Lister on the structure andf unctions of Polypi #Ascidise. 365 



March 6. — The reading of a paper, entitled, " On the Structure and 

 Functions of tubular and cellular Polypi, and of Ascidia?." By Joseph 

 Jackson Lister, Esq., F.R.S. — was commenced. 



March 13. — The reading of Mr. Lister's paper was resumed and 

 concluded. 



This paper contains the account of a great number of observations 

 made by the author during the last summer, while he was at the 

 southern coast of England, on several species of Serlularim, Plumu- 

 lar'ue, Tubularwe, Cumpanularice , Flustrce, and other polypiferous zoo- 

 phytes, and also on various Ascidia. Each specimen was placed for 

 examination in a glass trough with parallel sides, before the large 

 achromatic microscope of the author, directed horizontally ; and care 

 was taken to change the sea-water frequently, which was done by 

 means of two syphons, the one supplying fresh water, while the other 

 carried off the old ; a plan which succeeded in keeping the animals in 

 perfect health and vigour. The drawings which were taken of the 

 appearances that presented themselves were traced with a camera- 

 lucida, slid over the eye-piece of the microscope. 



In a specimen of the Tubularia indivisa, when magnified 1 00 times, 

 a current of particles was seen within the tube, strikingly resembling, 

 in the steadiness and continuity of its stream, the vegetable circula- 

 tion in the Chara. Its general course was parallel to the slightly 

 spiral lines of irregular spots on the tube ; on one side flowing from, 

 and on the other towards, the polypus, each current occupying one 

 half of the circumference of the tube. The particles were of various 

 sizes, some very small, others larger, but apparently aggregations of 

 the smaller : a few were nearly globular, but in general they had no 

 regular shape. At the knots, or contracted parts of the tube, slight 

 vortices were observed in the current ; and at the ends of the tube 

 the particles were seen to turn round, and pass over to the other side. 

 Singular fluctuations were also observed in the size of the stomach and 

 of the cavity of the mouth ; the one occasionally enlarging, while the 

 other contracted, as if produced by the passage of a fluid from the one 

 into the other and its subsequent recession, thus distending each alter- 

 nately. This flux and reflux took place regularly at intervals of 80 

 seconds ; besides which two currents were continually flowing, both 

 in the mouth and stomach ; an outer one in one direction, and an inner 

 one in the opposite direction. 



In all the species of Sertularia examined by the author, currents of 

 particles were observed passing along the soft substance which occu- 

 pies the axis of the stem and branches, and were even seen extending 

 into the substance of the polypi themselves, and traversing the sto- 

 machs belonging to each. Contrary to what happens in the Tubula- 

 ria, the stream does not, in these animals, flow in the same constant 

 direction ; but after moving towards one part for about a minute or 

 two with considerable velocity, it becomes much slower, and then 

 cither stops or exhibits irregular eddies, after which it resumes its 

 motion with the same velocity as before, but in the contrary direc- 

 tion ; and so on alternately, like the ebb and flow of the tide. If the 

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



