THE STRUCTURE OF XEN1A HICKSONI. 211 



polyps, the tentacles of which were devoid of pinnules, were youug 

 forms which had not yet attained the adult characters. 



In 1874 Kolliker described Heteroxenia Elizabethce, in which small 

 individuals are very numerous. He regarded these small individuals 

 as being of two kinds, some being young polyps in various stages of 

 development, others being "zooids." Heteroxenia, therefore, is dimor- 

 phic. According to Kolliker, there are several essential differences 

 between these two kinds of individuals. 



I. The Polyps. — These are of large size, the adult measuring 20 mm. to 

 55 mm. in length, and about 3 mm. in breadth. There are also obviously 

 younger polyps, 5 mm. to 20 mm. long, all of which are situated round 

 the edge of the disc. The tentacles of the adult polyps bear two series 

 (in each of which four rows are distinguishable at the base of the tenta- 

 cles) of long cylindrical pinnules, one on each side of the middle line of 

 the tentacle. The ccelentera of these polyps extend a considerable 

 distance into the stem of the colony, and are crowded with ova. 



II. The Zooids. — These are much more numerous than the polyps 

 and much smaller, measuring only 3 mm. to 5 mm. in length and from 

 •7 mm. to 1 mm. in breadth. The tentacles are eight short simple 

 lobes, •14 mm. to '2 mm. in length, and they bear no pinnules. The 

 ocelentera of these zooids extend at most only 3 mm. into the stem, 

 and contain no gonads. 



It should be noted that Kolliker saw two kinds of small individuals, 

 .and described the differences between them in size, structure, and posi- 

 tion, viz.: (1) the young polyps, 5 mm. to 20mm. long, found only 

 round the edge of the disc; and (2) the "zooids," 3 mm. to 5 mm. long, 

 found all over the disc among the bases of the larger polyps. 



Klunzinger (1877), who described the Xeniidos of the Red Sea, saw 

 small polyps in a specimen of Xenia umbellata ; he called them bud-like 

 polyps, and said that their tentacles which are at first simple, \erj soon 

 show indentations or pinnules. He regarded such individuals rather 

 as young polyps than as zooids. They were more numerous in the 

 outer part of the arched end of the stem. In a new species Xenia 

 fuscescens, Klunzinger described the small polyps as very numerous, 

 outnumbering the large polyps, and filling up the intervals lietvveen the 

 bases of the latter. He wrote that these small individuals do not 



