INTRODUCTION. 19 



the Zoophytes, and thus dispelled one error, presently com- 

 mitted a similar fault by transferring along with the Zoo- 

 phytes (simply on account of their agreement in certain 

 external characters), Coralhna and other genera which, it is 

 almost unnecessary to add, have not until a recent period 

 been restored to their proper place amongst the Alg8e*, The 

 above is a sufficient illustration of the danger of trusting 

 solely to the evidence of sight : I shall endeavour to show 

 that it would in the present case also lead to an incorrect 

 conclusion. 



That the Desmidiese have been so long associated in the 

 same family with the Diatomaceee, whose proper position is 

 so doubtful, presents another obstacle to the recognition of 

 the claims of the former to rank with the Algae. For when 

 so many eminent observers, — botanists and zoologists, not- 

 withstanding they differ widely in respect of the department 

 of Natural History to which these forms should be assigned, 

 — concur in classing them together, we naturally suppose that 

 there must be valid reasons for such a course. I shall 

 therefore point out the distinctions between these groups, 

 and show that, at all events, they can no longer be united in 

 one family, but must be separated, as they have been by 

 Kutzing in his more recent work, as well as by others. 



I have shown that the cell in the Desmidiese consists of 

 two valves united by a central suture, and that during its 

 division the new-formed portions are interposed between 

 these valves. The Desmidiese are membranous, or should a 

 few species contain silica, it is not present in sufficient quan- 

 tity to interfere with their flexibility. They rarely have 

 acute angles, and are seldom (if ever) rectangular. They are 

 often deeply incised or lobed, warted or spinous. The internal 

 matter is of a herbaceous-green colour, and starch vesicles 

 abound in the mature cell. They couple and form either 

 orbicular or quadrate seed-like bodies, and are remarkable 

 for the resistance which they oppose to decomposition. 



* See the introductions to Johnston's admirable works on the British 

 Zoophytes and Corallines. 



c2 



