DIATOMACE2E. 425 



in the side view they appear attached to the right and left 

 edges of the interior of the frustule. This colourless 

 granular substance carries in its centre, near the middle 

 part of the Diatom, an imperfectly developed nucleus 

 which it is not very easy to see, but may be easily de- 

 monstrated by the application of acid. The colourless 

 substance is what, in other Diatoms, Schultze shows to be 

 Protoplasm, or vegetable sarcode, and which contains 

 numerous small refractive particles ; on adding a drop 

 of a one per cent, solution of osmic acid these became 

 blue-black, and proved to be fat. It is, however, 

 exceedingly difficult to determine the exact limitations 

 of the protoplasm, on account of the highly refractive 

 character of the siliceous skeleton, and the obstruction 

 presented by the endochrome. 



After a short distance, the protoplasm reappears, and 

 is contracted into a considerable mass within the conical 

 terminations of the frustule. Schultze observed in this 

 part of the protoplasm a rapid molecular movement such 

 as is known to occur in the Closterium, and further, a 

 current of the granules of the protoplasm along the raphe. 

 Pleurosigma angulatum " crawls" as do all Diatoms pos- 

 sessing a raphe, along this line of suture. To crawl along, 

 it must have a lixed support. He believes free swimming 

 movements are never to be observed in this or in any 

 other Diatom. Accordingly, Schultze invariably found 

 that the raphe is in contact with either the glass slip or 

 the glass cover, between which the Diatom is placed, or is 

 in apposition with some foreign body of considerable size. 

 Schultze repeated the experiments of Siebold, and ob- 

 served, as he and also Wenham had done long before, 

 that particles of foreign matter stick to the raphe as 

 though it were covered with some glutinous material, 

 and are carried slowly along by the action of a current. 

 This he observed in many Diatomaceae, and found in- 

 variably that foreign particles adhered only to the raphe, 

 or what corresponded to it. "There is obviously," says 

 Schultze, " but one explanation ; it is clear that there must 

 be a band of protoplasm lying along the raphe, which 

 causes the particles of colouring matter to adhere, and 

 gives rise to a gliding movement For there is but one phe- 



