EVOLUTION WITHIN THE GENUS DENDRONEPIITHYA, 37 



III. The Uinbellatce are characterized by : — 



(a) the umbel-like or sometimes corymb-like aggregates formed 



by the terminal twigs, the heads of the umbels beino- 

 bundles of polyps ; ^ 



(b) the disposition of all or most of the polyp heads on the 



surface of the colony. 



Text-figure 3. 



A typical Umbellate. 



Thus the Umbellatas differ fi^om the Glomerata?, and agree with 

 the Divaricatse iu showing much minor branching. They differ 

 from the Divaricate in the presence of umbels or corymbs of 

 polyp bundles and in the entirely superficial arrangement of 

 the polyp heads. As this continuous superficial disposition 

 is obviously advantageous — giving all the polyps equal expo- 

 sure for nutritive and respiratory purposes — we may regaxxl 

 the Umbellatae as the latest expression of the evolution of the 

 Dendronephthi/a polypariura. Moreover, some Umbellatae pass 

 through a Divaricate stage. 



Of minor importance is the question whether or not the umbels 

 combine into large secondary bunches rising like hillocks on the 

 surface of the polyparium. Nor can I attach more than slight 

 importance to the shape of the polyparium whether flattened or 

 quite symmetrical, whether spherical or disc-like, and so on, for 

 these features j^robably depend to a large extent on local environ- 

 mental conditions. 



It may be noted that a large and veiy handsome colony of 

 jD. annectens was, as a whole, strikingly divaricate at first sight, 

 but a closer inspection soon showed that it was an Umbellate 

 having the terminal twigs massed into distinct umbels. (See 

 Plate III.) 



