760 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of colonies are then translucent and the direction of the currents pro- 

 duced by the cilia can be detected. The current formed by the cilia 

 of the filaments flows from the base of the polyp to the stomodaeum, 

 while that of the siphonoglyph is in the opposite direction. 



Growth. 



Growth being both terminal and radial, the polyps may increase 

 in either direction. The tips formed in summer are of two types. 

 One type shows no polyps on the terminal two or three millimeters 

 of the branch, which is crowded with purple spicules. In only a very 

 few instances were polyps formed at the tip of the stem in this type in 

 any other position than the radial one. They were usually large and 

 of the same size. This is not an area of reproduction of polyps at 

 this time. The other type of stem shows a tip denuded of polyps for a 

 relatively long region, one half to one or more centimeters. The 

 coenosarc wall of this tip is smooth and many of the polyps nearest 

 to the denuded region are small. Under the surface of this tip is 

 found an extensive network of canals. Very small polyps are also 

 often found in the coenosarc at other regions than that of the tip. 

 Young polyps, then, may be found in the growing stem in all parts 

 of the colony. 



Muscles and Nerves. 



The arrangement of muscles into systems is not markedly different 

 from that described for Alcyonium by Kassianow (:08). The systems 

 are: (1) The tentacle and disk system. This is ectodermal. The 

 muscle fibers (Plate 2, Fig. 5, my'nm.) run longitudinally on the 

 pinnules (Fig. 6) and on the tentacles (Figs. 5, 7) and are continued 

 on the disk toward the mouth, but the lateral strands of each of the 

 eight bands bend outward to be inserted in the mesogloea of the 

 mesentery. The median strand on the oral side of the tentacle is 

 continued to the mouth, but these muscles are fewer than in Alcyo- 

 nium. The aboral surface of the tentacle, as is shown by a transverse 

 section (Fig. 7; compare Fig. 21), bears no muscles, and muscles are 

 lacking on a very small portion of the aboral surface of the pinnules 

 (Fig. 6). (2) The polyp-wall system embraces muscles that are endo- 

 dermal and are arranged circularly (Fig. 20). They are strongest 

 where the polyp wall and tentacles meet, and they may pass a slight 



