THE STRUCTURE OF XENIA HIOKSONT. 289 



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

 of, and alteration in the character of, the tentacles, witii 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 dift'erent. 



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

 in nature from young polyps, and that there are in Hetero- 

 xenia two kinds of individuals, polyps and zooids, or, to use 

 Moseley's terms, autozooids and siphonozooids. 



The Internal Anatomy of Heteroxenia Elizabethee. 



Owing to the very imperfect preservation of the specimen, it 

 is impossible to give a detailed account of the internal struc- 

 ture 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. 



Autozooids.— Spicules are numerous in the ectoderm of 

 all parts of the body of the autozooid, and are very numerous 

 in the ectoderm of the outer face of tlie tentacles. They are also 

 present in the pinnules. They are similar in size and shape to 

 those of Xenia Hicksoni. They are whitish or bluish white 

 by reflected light, but of a reddish-brown tinge by transmitted 

 light. Nematocysts are exceedingly scarce and difficult to find. 

 They measure 9 ju in length and 2^ ju in width. The ecto- 

 dermic muscles of the tentacles are well developed ; the muscle 

 band of the oral face is very much thicker than that of the 

 aboral face. 



The oral disc of each polyp is slightly contracted, producing 

 a funnel-shaped depression leading to the mouth. The stomo- 

 d^eum is only I mm. to 1-5 mm. long, and is wrinkled or folded 

 transversely. It bears a ventral groove or siphouoglyph, the 

 cells of the lower half of which bear moderately long flagella. 

 There are the usual eight mesenteries, bearing rather feebly 

 developed retractor muscles on their ventral faces. Only the 



