764 CHARLES L. nOULENGER. 



sierteu Bildungsherdeu, von denen aus sie in eiuem bestimraten 

 Entwickelungsstadiutn als Wauderzellen auf die Verbraucbs- 

 stiitten ubervvandern." 



The subject has been recently revived by Jovan Hadzi (4) 

 in a remarkable paper in which he records his observations on 

 the thread-cells of marine hydroids. Hadzi's results are of the 

 greatest interest^ as he was able to examine the living tissues 

 as well as preserved material. His main conclusions are as 

 follows : 



(1) The thread-cells of hydroids are not formed "in situ'' 

 but in the ectoderm of the cccnosarcal branches, where, on 

 account of the thick perisarcal investment, they can obviously 

 not become functional, 



(2) When completely developed, except for accessory 

 structures such as the cuidocils and the stalks, they migrate 

 to the important nematocyst batteries on the tentacles. 

 This migration can take place in two different manners. In 

 simple forms, e.g. Campanularia, the thread-cells move 

 actively by means of their pseudopodia, making their way 

 between the ectodermal cells of the colony. In Tubularia 

 however, they adopt a quite different method of locomotion : 

 from the ectoderm of the ccenosarc they force a way through 

 structureless lamella and endoderm into the cavity of the 

 hollow stem, whence they are carried by the current caused 

 by the ilagella of the endoderm cells to the hydranths. 

 Here the thread-cells re-enter the tissues and migrate 

 actively by their own movements to the ectoderm of the 

 tentacles. 



In a recent paper, whilst describing the structure of the 

 Egyptian lacustrine medusa, Moerisia lyousi (5), I called 

 attention to the fact that large nematocysts were to be found 

 in abundance among the endoderm cells of the manubrium. 

 Being at a loss to account for their presence in this position 

 I cut sections of a large number of specimens, careful 

 examination of which convinced me that I was dealing with 

 a case similar to that investigated b}- Hadzi in Hydroids. 

 As this phenomenon has not been described previously in 



