AND SPECIES OE HYDEOIDA. 269 



and contained within the cage-like chamber formed by the spines 

 of the gonangium a well-preserved acrocyst. I have no know- 

 ledge of the form of the gonangia in the male ; but, judging from 

 analogy, they are probably destitute of the marsupial chamber 

 and acrocyst which characterize the female. This species is thus 

 distinguished by the peculiar form of its female gonangia, which 

 differ from those of the other described species of Thuiaria much 

 as the female gonangia of Biphasia differ from those of Sertularia. 



The specimen being imperfect, its actual height could not be 

 ascertained. About four inches of the stem remained ; but it had 

 evidently been broken off at some distance from the root. 



"When the gonangia are young, they are obconical in form, with 

 a broad, flat summit, and in this stage show no trace of mar- 

 supial spines. As the gonangium continues to increase in size it 

 assumes an obovate form, and the spines begin to grow out round 

 the margin of its summit. These are at first simple, and after- 

 wards become bifurcate. There are three bifurcations in every 

 spine, each branch of the first bifurcation dividing into two. 



An extension of the coenosarc is continued through the whole 

 length of the spines, from the enlarged summit of the blastostyle ; 

 and as the blastostyle must be homologically regarded as a hy- 

 dranth arrested and adapted to functions connected with repro- 

 duction instead of nutrition, I look upon the spines here in the 

 same light as I regard the corresponding parts in the gonangium 

 of Diphasia, namely as blastostylic tentacles, thus representing the 

 tentacles of a hydranth which have lost their prehensile functions, 

 become clothed with chitine, and adapted to the protection of the 

 ova during an early period of their development. 



The ova are formed as usual, in a sporosac which springs from 

 the blastostyle within the gonangium, and are subsequently dis- 

 charged into the marsupial chamber, where, however, they are 

 not free, but continue for some time confined within an acrocyst. 



Thuiaria bidens. Plate XVIII. figs. 1, 2. 



Trophosome. Hydrocaulus attaining a" height of 4 inches, 

 springing from an entangled mass of tubular filaments, much 

 and irregularly branched, pinnate, main stem and principal 

 branches fascicled for some distance from their origin, becoming 

 monosiphonic towards their extremities ; pinnae alternate ; hy- 

 drothecie of the pinnae adnate in their entire height, alternate, 

 following one another without an interval, somewhat swollen 



