THE STRUCTURE OF XENIA HICKSONI. 277 



current produced by the cilia of the dorsal mesenterial filaments. 

 Although escaping in a mass they are not enclosed in the 

 follicle, but lie loosely aggregated in the dorsal portion of the 

 stomodaeura. These two examples support the conclusion 

 expressed above that the spermatozoa are discharged into the 

 coelenteron by rupture of the follicle, the collapsed remains of 

 which retain for some time their attachment to the mesentery. 

 As spermatozoa are present in all stages of development, it 

 is likely that the discharge of ripe spermatozoa continues over 

 a considerable period, — in fact, probably throughout the year. 

 This is perhaps due to the fact that, living on reefs in the 

 shallow waters of tropical seas, this coral is not subject to any 

 great variations in temperature and food supply. In this 

 respect Xenia differs from Alcyonium digitatum, which 

 occurs in the colder seas of Northern Europe. In the latter 

 all the sperm sacs of a colony have reached a similar stage 

 of development and are all ripe about the same time of the 

 year, viz. December, and therefore the discharge of ripe 

 spermatozoa occurs only over a limited period, probably over 

 about a mouth (Hickson, 1895). 



Nervous System. 



In several sections there is a plexus of fine fibrils in the 

 mesogloea connected with very small cells in relation with 

 the ectoderm and endoderm. This plexus appears to be 

 homologous -with the similar plexus described by Hickson in 

 Alcyonium (1895, p. 371), and compared by him to the 

 *' Nervenschicht'^ of the Actinise. 



In this Xenia the plexus is best seen in sections of a polyp 

 in which the ectoderm is cut slightly obliquely. On examin- 

 ing in such a section (PI. 26, fig. 16) the part where the ecto- 

 derm passes into the mesogloea, very fine fibrils {N. F.) may be 

 seen, forming an open network, upon which cells (A^. C.) are 

 situated at intervals. The fibrils can be best seen in the 

 mesogloea, but can be traced close to the ectoderm and endo- 

 derm. The cells of the nervous system are exceedingly small, 



VOL. 42, PART 3. — NEW SERIES, T 



