ALCYONIUM. 103 



powerful cilia. This groove is the siphonoglyph (PL II., 

 fig. 6). The siphonoglyph indicates the position of the 

 ventral side of the polyp. 



The Mesexteries. — The mesenteries are in the greater 

 part of their course very thin, consisting of two layers of 

 endoderm cells covering a thin sheet of mesogloea. In 

 the region of the stomodseum, however, they exhibit longi- 

 tudinal thickenings or ridges which support the muscular 

 fibres that are principally concerned in the retraction of 

 the anthocodia. The arrangement of these thickenings 

 is very characteristic. They are all situated on the ventral 

 faces of the mesenteries, so that in a transverse section 

 the muscle ridges on the ventral mesenteries are face to 

 face, and on the dorsal mesenteries they are back to back 

 (Plate II, fig. 6). 



In the last six months of the year all the mesenteries, 

 with the exception of the two dorsals, bear a considerable 

 number of spherical bodies which are ova or sperm sacs 

 according to the sex of the colony (Plate III., fig. 20). 



The two dorsal mesenterial filaments run straight down 

 from the lower end of the stomodseum in \k the depths of 

 the polyp cavities. In the cavities of tiie older polyps 

 they may be traced almost to the base of the colony. The 

 epithelium covering these filaments is columnar and 

 ciliated, the cilia producing in life a current of water 

 flowing towards the stomodseum. The other mesenterial 

 filaments are of a perfectly different nature (PL II., fig. 

 15). They are much shorter than the dorsal filaments, 

 and are covered by an epithelium densely packed with 

 gland cells. Their function is to secrete a digestive juice 

 upon particles of food which pass through the stomodseum. 



The Endoderm lining tlie coelenteric cavity in the 

 anthocodia is composed of ciliated cubical cells closely 

 packed to form an epithelium. In the lower parts of the 



