2(i-J CEYLON PEARL OYSTER REPORT. 



downwards, so that the youngest ova usually occur nearest the lower end of the 

 stomodaeum. In a specimen of Sarcophytum latum, ova on a single mesentery vary 

 in size from 0'03 millim. to 0'5 millim. in diameter. Two series of developing ova, one 

 a little distance below the other, were observed on a single mesentery in Sarcophytum 

 contortion. 



On comparing the oogenesis of Sarcophytum, LobopJtytum, and Sclcrophytum with 

 ASHWORTH'S description of the spermatogenesis of Xenia, it would appear that the 

 sexually mature condition, in these tropical genera, extends over a considerably longer 

 period than in the case of corals inhabiting temperate waters. The complete absence 

 of male colonies, and of generative organs in many specimens, in the collection, is very 

 noteworthy. I can offer no explanation, for many of the colonies are of considerable 

 size, and the zooids appear to be mature in every other respect. 



In the Alcyonaria generative organs usually occur on the ventral and lateral mesen- 

 teries only (fig. 17). In a specimen of Sd-erophytwn gardineri, however, ova occur also 

 on the dorsal mesenteries (fig. 18). They are not connected with the ectoderm of the 

 mesentery, but are borne and supported by lateral outgrowths of the mesenterial 

 endoderm. ASHWORTH (1898, p. 207) records the presence of spermaria on the 

 dorsal mesenteries of Xenia, but these are apparently the only records of their 

 occurrence on these mesenteries in the Alcyonaria. As both forms in which they occur 

 present modifications in other respects, this must be regarded also as a secondary 

 feature, and of no special morphological interest. 



SIPHONOZOOIDS. 



I have slight additions to make to the description of the siphonozooids of Sarcophytum, 

 Lobophytum and Sclerophytum already given in a previous publication (PiiATT, 1903). 



In Sarcophytum and Lobophytum the siphonozooids are small in comparison with 

 the size of the autozooids, but in Sclerophytum they are either very minute (fig. '20) 

 or absent. In all well marked cases of dimorphism the stomodasum is large, usually 

 ciliated throughout, and has a well marked siphonoglyph furnished with long flagella 

 (fig. 19). The dorsal mesenteries are long and have large, grooved and ciliated 

 filaments. Even when of considerable size siphonozooids are usually only slightly 

 contractile, and have only small ventral mesenteries. The specialisation of these 

 zooids to fulfil a circulatory function has been accompanied by a reduction and 

 atrophy of those organs which are concerned in the capture and digestion of food. 

 Throughout the Alcyonaria siphouozooids apparently have no tentacles and their 

 ventral mesenteries no filaments. The increase of the flagellate surface of the 

 rtomodmun has been at the expense of the digestive surface. The increased current 

 of sea water within the zooids would carry away the food particles so rapidly, that 

 they would be unable to receive the secretion of the gland cells, which, however, 

 would Ix: rendered so dilute as to be quite useless for digestive purposes. These 



