276 E. RAY LANKESTER. 



servative medium, Glycerine. This latter fluid is caused to 

 flow under the glass cover by means of absorption along 

 one of its sides, produced by a piece of blotting paper. 



In the hearts of mature full-grown A. furcata the 

 structure is a little more complicated, as seen in figs. 

 7, 8. In fig. 7 the fibrillar part of the heart is obscured 

 by the oesophagus which runs across this particular part, 

 and has not been displaced in the slide from which the 

 drawing has been made. Fig. 8 shows but one heart-cell, 

 with its connected fibrils. There are fhirteen fibrils in 

 the former of these hearts, and twelve in the latter. The 

 variation in their number can have no significance. In 

 fig. 7 they are most clearly seen to arise from one edge only 

 of the broad base of each heart-cell, and to leave the rest of 

 the margin free. As an addition to what was observed in the 

 smaller heart, there are now present small secondary cor- 

 puscles, lying at the base of the fibrils, sometimes between 

 two, sometimes closely embracing two or three. These 

 small secondary corpuscles {s, s) are not nucleated cells, and 

 it is not easy to form an idea as to the mode of their origin. 

 They are not indicated in the earlier condition of the heart, 

 and it is clear that they are not the morphological equivalents 

 of the two large conical heart-cells. One cannot be sure that 

 the condition represented in figs. 7, 8 is the final, adult 

 heart, — a strange reduction of that organ if it be so, since it 

 has not even a tubular structure or cavity, still less are 

 there vessels connected with it. It is simply a most vigorous 

 churn, beating and stirring up the fluid in the great peri- 

 visceral hsemolymph space without propelling it in any 

 particular direction. 



The reduction of the number of the constituent cell- 

 elements or plastid-units in such small organisms of elaborate 

 organisation as are the Appendicularise and the Rotifera, 

 may help to make clear some of the processes of growth and 

 development in organisms generally. To what extent can 

 this reduction be carried ? May we not possibly even arrive 

 at a stage in retrogressive metamorphosis where cells no 

 longer differentiate at all ? Are we not prone to assign too 

 important an office to the plastid, even as an element in com- 

 plexity of organisation ? The living matter of the organism 

 is what develops and elaborates structure; its segregation 

 into a greater or less number of corpuscles is a simple eff'ect 

 of the relations of bulk and cohesion. At the same time the 

 limits of size on the side of minuteness which can be presented 

 by the higher types of organisms seem to be determined by 

 the impossibility of reducing the number of constituent units 



