366 MR. LISTER ON THE STRUCTURE AND FUNCTIONS 



much nearer to the Ascidice than to the Tubularice or Sertularice with which some of 

 them have been hitherto associated. 



It may deserve to be mentioned^ that as the species of animals of one type diminish 

 in size, the delicacy of some of the remoter details of their structure does not increase 

 in the same proportion ; but the aggregation of tlie component parts in the smaller 

 species becomes instead more simple. From this cause, as well as from their trans- 

 parency, most of the objects examined offered peculiar advantages for inspecting 

 their organization while living and in freedom. They were placed in a glass trough 

 with parallel sides, before my achromatic microscope directed horizontally, and the 

 sea water was often changed. Near the end of the observations this was done by 

 two siphons, one of which constantly admitted a fresh supply, while the other carried 

 off the excess ; a mode which, had it been earlier adopted, might have rendered some 

 of the results more satisfactory ; for the great difficulty, next to procuring a variety 

 of specimens, was to retain them in vigour. 



With the exception of two species that grew above the line of low water at spring 

 tides, those obtained were only what the waves threw up ; for repeated endea- 

 vours to get them, through fishermen, from rocks at sea or from oyster-grounds were 

 unsuccessful. 



The drawings in illustration were traced by a camera lucida slid over the eyepiece 

 of the microscope. The facility with which correct graphic records and measurements 

 may be obtained by means of that instrument with a little practice, induces me to re- 

 commed its use to other observers. The linear enlargement is marked to each figure. 



TUBULAR POLYPI. 



TUBULARIA. 



The specimen of T. indivisa figured Plate VIII. fig. 1 ., was one among a broken mass 

 of tubes, most of them larger than itself, which was found at Dover, October 1832, 

 freshly cast ashore, and was kept several days, on another account, before the polypus 

 terminating it was observed. It is a very small example of the species to which it is 

 considered to belong. The arms had no cilise. 



When magnified about one hundred times, a current of particles was seen within 

 the tube, that strikingly resembled, in its steady continued flow, the circulation in 

 plants of the genus Chara. The general course of the stream was parallel to the 

 slightly spiral lines of irregular spots on the tube, and in the directions marked by 

 the arrows. On the greater part of the side first viewed (that shown in the draw- 

 ing), it set as from the polypus ; but on reversing the glass trough so as to show the 

 other side, the flow was there towards the polypus ; each current thus occupying half 

 of the circumference. The particles had no dancing motion among themselves like 

 those which will be hereafter mentioned in Sertularia, hut floated evenly on at a uni- 

 form rate. They were various in size ; some very small, others apparently aggrega- 



