THE STRUCTURE OF XENIA HICKSONI. 209 



edge, so that, although the sac is of considerable size ('15 mm. in 

 diameter), it does not push the dorsal mesenterial filament out of 

 position, and, judging from the sections, would not impede its action. 



Empty sperm sacs the walls of which are collapsing may be seen in 

 several sections. The spermatozoa are discharged into the coeleuteron by 

 bursting of the follicle of the sperm sac, and are then swept out to the 

 exterior through the stomodgeum. In sections of two polyps in which 

 spermatozoa were escaping, the spermatozoa are fonnd along the dorsal 

 side of the stomodBeum, being doubtless driven out by the upward or out- 

 ward 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 position of the stomodseum. These 

 two examples support the conclusion expressed above that the sperma- 

 tozoa 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 spermatoza 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 month (Hickson, 1895). 



Nervous System. 



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

 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 Actinice. 



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

 the ectodern is cut slightly obliquely. On examining in such a section 

 (PI. XXL, Fig 16) the part where the ectoderm passes into the meso- 

 glcea, very fine fibrils (N.F.) may be seen, forming an open network, 



N 



