Royal Physical Society. 265^ 



in which the polyp was in course of being absorbed, and its 

 solid matter stored in the circulating fluid for the production 

 of its successor. 



On first obtaining a favourable specimen of Tubularia in- 

 divisa, I directed my attention to the structure of the polypary, 

 and the phenomena of the circulation within it. I found that 

 each of the spiral lines was generally formed of two narrower 

 lines running close and parallel to each other (Plate XV., fig. 

 3), and that the circulation took place along the wider interval 

 between the double lines. These intervals had the appear- 

 ance of canals situated immediately beneath the corallum, 

 and occasionally communicating with each other by cross 

 branches. A thin transverse section of the stalk, readily made 

 by the aid of a fine pair of scissors (Plate XV., fig. 2), showed 

 that (with the exception of a thin layer of " ectoderm" b, which 

 lined the inside of the corallum a), the whole of the tube was 

 filled with a highly-vacuolated or cellular " endoderm'' c, 

 having the appearance of the pith in the section of an exo- 

 genous plant, and was generally impervious to the passage 

 of fluid. Immediately within the ectoderm, the endoderm was 

 perforated by eight or more equidistant canals d, finely ciliated 

 in their interior, and having their walls loaded with coloured 

 granular matter. The interstices between these canals corre- 

 sponded to the double lines seen in the longitudinal view (fig. 3). 

 As the polypary emerged from the corallum, the tubes became 

 wider, and opened into each other until they formed a single 

 cavity immediately beneath the lower range of tentacles of 

 the polyp ; here the circulation became influenced by a mecha- 

 nical provision, hereafter to be described. The circulation in 

 Tubularia indivisa, therefore, as far as relates to the polypary, 

 is carried on by ciliary motion in canals which permeate the 

 periphery of the endoderm in the longitudinal direction of 

 the stem. The movements in the different canals are not 

 related ; in some of the canals the fluid is passing upwards, in 

 others downwards, and in others it is at rest, previous to its 

 commencing to flow in an opposite direction. 



The polyp of Tubularia is distinguished by two rows of 

 filiform tentacles,— the one short and fringing the mouth, the 



