the structure of xenia hicksoni. 227 



General Summary. 



The following is a recapitulation of the new points to which reference 

 is made (paragraphs 1, 2, and 6 have been already published in ' Proc. 

 Eoy. Soc.,' 1898) : 



1. The absence of ventral and lateral mesenterial filaments usually 

 present in the polyps of the Alcyonaria. The only previously recorded 

 examples of the absence of these filaments are the siphonozooids which 

 occur in some other Alcyonacea and in Pennatulids. 



2. The absence of these filaments is correlated with the presence of 

 gland cells in the stomodeeum, which occur especially in the ventro- 

 lateral walls which abut on the siphonoglyph. Their position is sug- 

 gestive of the digestive function of the cells, as their secretion can be 

 readily poured out on to the ingoing food particles. 



3. The non-retractile nature of the bodies of the polyps and of the 

 stems is accounted for by the absence of muscle-fibres from their ecto- 

 derm cells, aud by the presence of numerous spicules in these parts. 

 The presence of ectodermic muscles in the tentacles, pinnules, and 

 distal millimetre of the bodies of the polyps, together with the absence 

 of spicules from these parts, confers the power of contractility, slight 

 though it is, upon these parts. The muscle processes of the ectoderm 

 cells (where present) are longitudinal in direction, while those of the 

 endoderm cells are circular (except the protractor and retractor muscles 

 on the mesenteries). 



4. Nematocysts were found in Sarcophyton. 



5. An extraordinary number of spicules is present in the basal part 

 of the colony, and much of the mesoglcea is converted into a dense 

 horny substance. Thus a firm base of attachment is provided which 

 would afford a rigid support for the branches which arise from it. 



6. Many of the endoderm cells lining the ccelentera and tentacles 

 bear giant flagella, which may attain 120 /j. in length. 



7. In adult polyps the primitive genital cells are formed by differ- 

 entiation of some of the endodei'm cells which cover the mesenteries. 

 These genital cells migrate into the mesoglcea of the mesenteries, and 

 then move outwards, one at a time, each cell pushing the endoderm 

 and a thin film of mesoglcea before it, and so forming a small tubercle 

 on the side or end of the mesentery. By division of the genital cell 

 the spermatozoa are produced. They remain until ripe, surrounded by 



