Crosby : OBSERVATIONS ON DicTvospiiyERiA. 63 



Mounting Media. 



1. Glycerine jelly necessitated a second change from gum- 

 arabic. 



2. Pure glycerine proved too strong drew out the stain and 

 clouded when heated. 



3. Formaline proved best. 



Habitat. Dictyosphceria favulosa occurs in all tropical seas, 

 /'. e., Hawaiian Islands, Grenada, St. Thomas, Barbadoes, 

 Ceylon, Mauritius, Red Sea and Philippine Islands. 



In all cases it was found firmly attached by rhizoids to flat- 

 tened coral reefs, as smaller, more rounded plants, or as larger 

 somewhat appressed areas. In shallow water at low tide it 

 grows attached to outer surface of reef or sides of hole in same. 

 It is often mixed with or covered by other algae. 



GROSS ANATOMY. 



A typical older thallus consists of an irregular, flattened, hol- 

 low hemisphere, with a single layer of large closely appressed, 

 hexagonal cells, enclosing a hollow center; attached to sub- 

 stratum by central rhizoids on lower surface {Fig. /). The 

 thallus, in early stages somewhat bag-like, later flattens and 

 becomes irregular in shape. 



Owing to the plasticity of such an undifferentiated thallus, 

 the size and shape of both surfaces are adapted to the position ; 

 expanded if attached to a flat surface ; more bag-like in form 

 with wedge-shaped base, if placed between two surfaces. Later 

 the flattening of the upper and lower surfaces, and the irregu- 

 larity of the upper surface arise as follows : 



In younger solid plants the cells are of equal size. Soon 

 those in the center enlarge and through the growth of outer cells 

 become torn and disorganized (PL XV., Fig. 4). The hollow 

 thus formed enlarges by the same process. The thallus lacks 

 cohesion, gained by interlacing branches in Struvea, and is 

 bound together by a membrane ; this now splits in all directions 

 causing the thallus to rupture. The membrane, mentioned by 

 Harvey, Kiitzing and Murray, as extending over the plant jbody 

 in younger stages, was not found. 



The cells divide continuously and replace the torn tissue by a 

 single-celled layer. The resultant form is irregular, expanded, 

 with hollow center, enclosed by upper and lower surface layers, 

 often filled with water (PL XV., Fig. 2). The outer surface of 

 the cells is tough and membrane-like. 



