128 REPORT ON THE SPECIMENS OF THE GENUS MILLEPORA 



The result of this observation was to prove that in many of the fully exploded 

 nematocysts a faint spiral line runs through the whole length of the thread. 

 Considering the fact that some of the nematocysts collected in this way may have 

 been shot a long time before they were preserved it is not surprising that some of 

 the threads do not stain at all, but the observed facts are quite consistent with the 

 view that a myophan fibril does run the whole length of the large nematocyst thread 

 in Millepora. 



I may point out in this place that the figure given by Moseley (il) of the 

 large nematocyst of Cerianthus bathymetricus shows a fibril running up the lumen 

 of the thread very similar to that which I have drawn in Fig. 2, but I can find 

 no statement in this paper or in others which I have consulted as to the retraction 

 of the thread. 



Schulze, Allman and others have described fine processes proceeding from the 

 base of the cnidoblast towards the deeper parts of the ectoderm, which Claus, Chun 

 and Hamann considered to be probably muscular in function 1 . 



Thinking that the retractility of the nematocyst observed by Dr Willey might 

 possibly be due to the cnidopod, as Allman calls these processes, drawing back the 

 whole nematocyst, I examined the sections of the best preserved material in my 

 possession again very carefully to see if such processes occur in connection with 

 the cnidoblast in Millepora and arrived at the following conclusions. The nernatneyst 

 is enclosed in a cnidoblast provided with an oval nucleus. Pseudopodial processes of 

 the protoplasm often occur penetrating between the walls of neighbouring cells. 

 These processes do not differ in structure from other parts of the protoplasm of the 

 cnidoblast and are consequently not specialised myophan strands (PL XVI. Fig. 5). 

 The nematocysts may occur either in the ectoderm or in the endoderm, and probably 

 migrate from the deeper parts of the canal system to the surface according to the 

 needs of the colony. There is no cnidocil, and the nematocysts may be exploded before 

 they reach the surface (PI. XVI. Fig. 4). 



Moseley originally pointed out that the large ovoid nematocysts occur in the canal 

 system in positions at a considerable depth from the surface, but apparently thought 

 that they were always in the ectoderm. The thin sections which we can now prepare 

 in great numbers have convinced me that they also occur in the endoderm, and in 

 all probability migrate to the surface when necessary. The fact that in some of my 

 sections a large number of exploded nematocysts are found at a considerable depth 

 from the surface is of importance as proving that nematocysts are not always exploded 

 by the trigger action of a cnidocil. In these cases the explosion must be due to a 

 stimulus brought from a distance by the nervous system and not to a direct irritation 

 of the cnidoblast. 



Size of the Nematocysts. The vesicle of the large nematocysts is in all cases 

 approximately - 02 mm. in length. In every specimen there will be found slight 



1 I have not thought it necessary to discuss more fully these points in the anatomy of the cnidoblasts, 

 as the reader may find a full statement of the views on the subject in Allmann's Challenger Report on the 

 Hydroida n. pp. xiv — xvii, and in the more recent paper of von Lendenfeld, " Die Xesselzellen der Cnidaria." 

 Biol. Centralbl. xvii. July 1897. 



