260 



CEYLON PEAEL OYSTER REPORT. 



durum. In this species they are extremely difficult to see, and are only about 

 5/u, long and 2/z, broad. 



Throughout the family the thread is apparently unbarbed ; nematocysts of Alcyo- 

 nium with discharged threads are figured by Hickson (1895). The nucleus of the 

 cnidoblast is usually long and curved (fig. 13), and stains very readily. Nematocysts, 

 similar to those occurring in batteries in the tentacles, are frequently to be found in 

 the ectoderm of the mouth disc, in the stomodseum, imbedded in the peripheral 

 portions of the mesenterial filaments, in the endoderm of the canals, and in some cases 

 in the mesoglcea some little distance below the surface. They are extremely rare in 

 the ectoderm covering the general surface of the colony between the zooids. 



Tentacles. 



"When the tentacles of Alcyonium are expanded, their ectoderm is extremely thin, 



and is composed almost entirely of batteries of cnidoblasts, columnar interstitial cells, 



scleroblasts, and a few mucous cells. I have observed no granular gland cells such as 



occur on the mouth disc. In the ectoderm of the body wall, nematocysts are 



comparatively few in number, and are rarely seen in the ectoderm covering the 



colony. 



ZOOCHLORELI^E. 



Zoochlorellse are more or less fully described in a publication now in the Press 

 (Pratt, 1905). In a specimen of Sclerophytum densum some of the zoochlorellae 

 are seen to have lost their cellulose cell walls (fig. Lb). In such a case the 

 nucleus, chromatophore, and protoplasm of a cell exhibit unmistakable signs of 



