4 liRITISH DESMIIMACK.E. 



granules and warts, many species of this genus are 

 adorned with spines of variable length, or the angles 

 of the semicells are produced into long hollow 

 processes. Seen from the vertical view, many exhibit 

 a radiating structure, the number of radii varying 

 from three to ten. The two genera Xanthidium and 



also contain some of the most beautifully 

 ornamented Desmids, but perhaps the prettiest of all 

 forms are to be found in the genus Micrasterias, the 

 exquisite symmetry of the incised margins of species 

 such as Micrasterias apiculata, M. r<i<H<>s, and M. fur- 

 cat* i, being without parallel in the vegetable kingdom. 



The great diversity of form and wonderfully varied 

 character of Desmids are features associated with their 

 almost exclusive confinement to small ponds or the 

 quiet margins of lakes, localities suitable for their 

 existence in large numbers. The complexity of out- 

 line, which is so frequently accompanied by a defensive 

 armour of spines and spinous processes, has most 

 probably been acquired as a means of resisting the 

 attacks of aquatic animals, such as Amoeba?, Turbel- 

 laria, Oligochastes, Tardigrades, small aquatic larvas of 

 insects, and small species of Crustacea; and it is note- 

 worthy that most species which occur on wet rocks and 

 in other localities from which these enemies are almost 

 entirely absent, possess as a rule a comparatively 

 simple outline. 



The cell-wall is composed of cellulose with a vari- 

 able admixture of pectose. The pectose compounds 

 are gelatinous, and in most Desmids they accumulate 

 as a considerable mucilaginous mass on the outer 

 surface of the cellulose Avail. It is most probable that 

 this accumulation is due to a gradual exudation of 

 these gelatinous compounds through numerous minute 

 pores present in the cellulose wall of almost all 

 Desmids. This is largely borne out by the radiating 



O e/ t/ 



fibrillar structure which is so characteristic of this 

 mucus and which has at times caused serious errors of 

 observation. The pectose compounds sometimes form 



