THE STRUCTURE OF XENIA HICKSONI. 249 



in the middle. They are arranged more or less in two series, 

 one on eaeh side of the middle line of the tentacle^ but three 

 rows on each side are not distinguishable ; at a distance of 

 about 1 mm. from the tip of the tentacle, however, the typical 

 arrangement of two series, with three rows of pinnules in each, 

 is gradually assumed (fig. 3, B). 



At the base of the tentacle the pinnules are much smaller 

 but have the typical arrangement, except in the case of one or 

 two of the proximal transverse rows. Here the pinnules appear 

 to be in course of formation, the outer ones being formed first, 

 the inner ones developing from without inwards (see fig. 3, D). 



On looking at the tentacle from the outer side only the 

 outermost row of pinnules is usually visible. These, which are 

 about twelve to twenty in number on each side, are set close 

 together and point towards the tip of the tentacle (fig. S,A). 

 At the tip of the tentacle the arrangement of the pinnules may 

 be well studied, and, as in Alcyonium (Hickson, 1895), they 

 are not paired (PI. 24, fig. 2). The pinnules are conical eleva- 

 tions with rounded ends. Those in the middle portion of 

 the tentacle are about '5 mm. long and ']5 mm. to "2 mm. 

 broad, but those nearer the base and tip are smaller. The 

 pinnules when fully expanded are about three times as long as 

 they are broad, and each tapers gradually from its base to its 

 blunt, rounded tip. When slightly contracted the pinnule is 

 somewhat swollen at its base, and if further contracted becomes 

 more swollen and globular at its base as its length decreases. 

 Although the body of the polyp is non-retractile, the tentacles 

 are often found slightly contracted, being in many cases curled 

 inwards over the mouth. Several examples of tentacles in this 

 condition are shown in fig. 1. 



Diagnosis of the Species Xenia Hicksoni. 



The species of Xenia are distinguished from each other by 

 the general form of the colony, the size of the polyps and 

 tentacles, the number of rows and shape of the pinnules, and 

 the presence or absence of an area free from pinnules on the 

 inner face ot the tentacle. 



