ORIGIN AND SIGNIFICANCE OF SPINES 43 



diatoms, the external relations are found to be A'ery much 

 like those of animals. The frustrule of the diatom AttJwya 

 decora ^'' is quadrate in outline, and from the angles there 

 extend sharp spinous processes, as represented in figure 32. 

 The frustrule of the desmid Staurastrnm cuspidatum is com- 

 posed of two triangular halves, and the spines project from 

 the vertices of the angles. Other species of Staurastrum, 

 Xanthidmm (X. armatiim*''}, Arthrodesmus (A. octocornis^^'), 

 etc., sliow similar spine growth from the most prominent 

 portions of the frustrules. It is evident that in forms like 

 these having angular outlines, any growth produced by exter- 

 nal stimuli will naturally be greatest at the points of these 

 angles, and in conformity with the previous analyses of these 

 factors a spiniform extension of the tissues would result. 



Among the freshwater Rhizopoda belonging to the Pro- 

 toplasta (= Amoehind^ the genus Difflugia affords good 

 examples. D. glohulosa*^ has a nearly spherical shell. In 

 D. pyriformis *^ the shell is elongate pear-shaped, and gener- 

 ally round on the summit or fundus, though in rare instances 

 a central spiniform elevation is developed. This tendency 

 becomes fixed in D. aeuminata*^ in which the shell in general 

 form resembles the preceding species, but the fundus is 

 commonly prolonged into a single acuminate process (figure 

 3-3), though occasionally two or three spines are found. In 

 D. corona " there is a circlet of spines around the margin of 

 the fundus besides the primary one in the centre. Difflugia 

 constricta *i is a variable form, with the top of the shell gen- 

 erally smooth, though sometimes it is acuminate, and occa- 

 sionally it has two or even a cluster of spines (figures 34-36). 

 Euglypha mucronata^^ has a terminal spine, as in JJifflugia 

 acumimdaA^ Placocista spmosa " is a flattened mitre-shaped 

 form with a distinct edge, along which are numerous spines. 

 It should be noted that no spines are developed on any 

 portion of these freshwater Rhizopoda except those here 

 mentioned. 



The Nasellarian Radiolaria furnish many instances of a 

 terminal spine from the summit of the silicious helmet or 



