ORIGIN AND SIGNIFICANCE OF SPINES 43 



diatoms, the external relations are found to be very much 

 like those of animals. The frustrule of the diatom Attheya 

 decora 47 is quadrate in outline, and from the angles there 

 extend sharp spinous processes, as represented in figure 32. 

 The frustrule of the desmid Staurastrum cuspidatum is com- 

 posed of two triangular halves, and the spines project from 

 the vertices of the angles. Other species of Staurastrum, 

 Xanthidium (X. armatum 47 ), Arthrodesmus (A. octocornis 59 ), 

 etc., show 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 (= Amoelina), the genus Difflugia affords good 

 examples. D. globulosa* 1 has a nearly spherical shell. In 

 D. pyriformis 41 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. acuminata 41 in which the shell in general 

 form resembles the preceding species, but the fundus is 

 commonly prolonged into a single acuminate process (figure 

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

 D. corona 41 there is a circlet of spines around the margin of 

 the fundus besides the primary one in the centre. Difflugia 

 constricta 41 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 41 has a terminal spine, as in Difflugia 

 acuminate.* 1 Placocista spinosa* 1 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 



