740 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Methods. 



Small colonies were kept alive in large aquaria of running watei' 

 for a short time. Small tips, 5 to 10 cm. long, were easily kept in 

 smaller dishes of nmning water, if care was taken to keep them 

 upright. Both Bouin's fluid and a five per cent formalin were suc- 

 cessfully used as fixing agents. The fixing fluids were taken in some 

 instances to the reefs and large tips or other pieces cut off from the 

 colony and quickly transferred to the fluid; upon returning to the 

 laboratory these were cut into small pieces. Both neutral formalin 

 and Vom Rath's picric-osmic-acetic-platinic chloride fluid were used 

 for nerve fixation. Corrosive sublimate and Wilson's fluid were 

 both used, but did not give any better results than the formalin or 

 Bouin's fluid. Decalcification was effected by 1 % acetic acid in 

 absolute alcohol. Maceration by Hertwig's method gave good 

 results. Delafield's haematoxylin and iron-alum haematoxylin could 

 be used with decalcified material. Many sections of the soft tips 

 were made without decalcification; but in these the haematoxylin 

 overstained the spicules and the axial skeleton, obscuring the neigh- 

 boring cells. By far the best general stain for these was Mallory's 

 phospho-tungstic haematoxylin, which stained well after formalin 

 and better after Bouin's fluid. This has the advantage that, while it 

 does not obscure the spicules, it differentiates other structures. 



General Structure, 



A colony of Pseudoplexaura crassa presents a loosely divided group 

 of long branches on a short stem; or the short stem may have short 

 branches which divide and divide again, terminating in long whips. 

 A drawing of a very young colony (Fig. A) illustrates the character 

 of the branching. The short basal shaft spreads out on the coral 

 rock or on old coral masses, secreting a skeleton that becomes very 

 firmly attached to its substratum. The whips, which rise in many 

 planes, are cylindrical, long and flexible, and taper somewhat gradu- 

 ally to the tips, which, however, end bluntly, or even with a slight 

 enlargement. The brownish polyps stand at right angles to the 

 branches (Plate 1, Fig. 1), closely crowded against one another, except 

 at the tips of the branches, where the coenenchyma can often be seen 

 even when all the polyps are expanded. Because the polyps when 



