THE STRUCTURE OF XENIA HICKSONI. 219 



By means of their rudimentary, short, rounded tentacles the zooids 

 may be readily distinguished from young polyps of the same size, whose 

 tentacles are longer, pointed, and bear pinnules. Compare, for example, 

 the young polyp A 5 (see table, p. 218) with the siphonozooid S 5. 

 They are both about the same length, but the tentacles of the former 

 are 1*3 mm. long, and, examined from outer aspect, show seven 

 pinnules on each side of the middle line, whereas the tentacles of the 

 siphonozooid are only "2 mm. long, and are simply rounded lobes. 

 Now if, as urged by Schenk and others, these two individuals are 

 both young polyps, how can the differences in the size and character of 

 their tentacles be accounted for? If the zooids are stages in the 

 development of polyps, it is very difficult to account for so many being 

 in the same stage of development (as the tentacles of all the specimens 

 examined are in the same simple condition), when, on the same colony, 

 other individuals of the same length, or even smaller, have already 

 acquired some of the adult characters, viz., the pinnate tentacles. On 

 the colony drawn in Fig. 37 there are over two hundred zooids, all of 

 which are similar in appearance, having simple round tentacles. The 

 examples indicated in the above table are chosen haphazard from 

 this large number, being arranged in order of length merely for 

 convenience of reference If these were young polyps we should 

 expect to find a more or less constant increase in the length of, and 

 alteration in the character of, the tentacles, with the increase in length 

 of the individuals ; but this is not the case. We should also expect 

 them to resemble other individuals of the same length on the colony 

 which are undoubtedly young polyps ; but, as we have seen, they are 

 very different. 



It must be concluded, then, that these zooids are different in nature 

 from young polyps, and that there are in Heteroxenia two kinds of 

 individuals, polyps and zooids, or, to use Moseley's terms, autozooids 

 and siphonozooids. 



The Internal Anatomy op Heteroxenia Elizabeths. 

 Owing to the very imperfect preservation of the specimen, it is im- 

 possible to give a detailed account of the internal structure of the 

 colony, and therefore attention will be chiefly directed to the points in 

 which Heteroxenia differs from the Xenia described in detail in the 

 former part of this paper. 



