THE STRUCTURE OP XENIA HICKSONI. 279 



the Xeniidse. They noticed them in Cornularia viridis, 

 which appears to belong to the genus Xenia, and they 

 suggested that these small polyps, the tentacles of which were 

 devoid of pinnules, were young forms which had not yet 

 attained the adult characters. 



In 1874 Kolliker described Heteroxenia Elizabethae, 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 dimorphic. According to Kol- 

 liker, there are several essential differences between these two 

 kinds of individuals. 



I. The Polyps. — These are of large size, the adults measur- 

 ing 20 mm. to 55 mm. in length, and about 3 ram. 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 tentacles) of 

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

 the tentacle. The coelentera of these polyps extend a consider- 

 able 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 ram. to 2 mm. in length, and 

 they bear no pinnules. The coelentera 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 position, viz. : (1) the young polyps, 5 mm. to 20 

 ram. long., found only round the edge of the disc ; and (2) the 

 " zooids," 3 ram. to 5 mm, long, found all over the disc among 

 the bases of the larger polyps. 



Klunzinger (1877), who described the Xeniidse of the Red 

 Sea, saw small polyps in a specimen of Xenia umbellata; 

 he called thera bud-like polyps, and said that their tentacles, 



