94 



ANDREWS. 



flavicans, which has been variously described from time to time. 

 Earlier observers thought the cuticle to be decorated with a 

 beaded formation in the peripheral protoplasm. Greeff later 

 ascribed the optical appearance to two sets of muscular fibrils 

 surrounding the creature and crossing each other more or 

 less at right angles. He thought that these two sets of fibrils 

 contracted alternately as the animal extended, or shortened, 



itself. 



I find the true structure to be that of a large cuticular layer 

 of vesicles of Biitschli's structure, quite uniform in size, and 

 covered by a pellicle of protoplasm which in its turn shows a 

 peripheral layer of delicate alveoli of the finer foam, (now for 

 the first time described). The interalveolar substance of the 

 cuticular layer is much modified from mere lamellar substance. 

 It contains strands, or plates, of the finer foam, which pass 

 rhythmically from viscid to fluid states, showing from minute 

 to minute a host of local, intermediate states and modifications 

 of its structure. Such modification of the interalveolar sub- 

 stance is along lines in one plane, and in two directions. The 

 lines lying between the alveoli intersect each other at such 

 angles as the shape of the body directs. 



When the animal was in medium extension, the optical stria- 

 tion was about as marked in one set of lines as the other, in 

 neither so marked as during more extreme change of shape. 

 The effect was then somewhat strongly that of the muscular 

 fibrils Greeff described. If greater extension occurred, that 

 set of lines parallel with the increasing diameter grew thicker, 

 and gained in optical emphasis. At the same time, the other 

 set of strial lines, running parallel to the decreasing diameter, 

 lost optical distinctness by becoming thinner and thinner as 

 extension proceeded. When the animal contracted, the phe- 

 nomena were reversed. But still the loss of emphasis was 

 always parallel with the decreasing diameter ; and gain in 

 optical and physical definiteness was always parallel with the 

 increasing diameter. 



If Greeff' s idea were the true one, and the striae represented 

 two sets of muscular fibrils, quite another set of appearances 

 would be seen, for then those striae corresponding with the 



