Points in the Histology of C<jetenterates. 261 



subsequent paper). I tlicrcfore believe that, starting from 

 the cavity of the cai)sule, which has previously been t"ortne<l 

 ■with the inner wall of the subsequent perfect cyst, the deve- 

 lo))nient of the thread [)roceeds in the protoplasm of the cnido- 

 blast, and that after completion the thread is introverted, 

 commencing with the tip and ending with the thickened basal 

 portion, so that this enters the caj)sule last. The develop- 

 ment of the outer wall of the capsule is the last to take place, 

 and, as it ap|)ears to me, does not occur until the spot is 

 reached at which the cyst comes into operation. The 

 extremely interesting formation of the thread, from its histo- 

 logical aspect, will be described in my subsequent paper. 

 The lamellar arrangement in the protoplasm of the cells of 

 the nettle-pad, as described by the brothers Ilertwig *, and 

 as is actually seen in animals macerated in a mixture of osmic 

 and acetic acid, is due to the disposition of the thread around 

 the wall of the nematocyst. I did not clearly grasp this 

 point until I examined the pad, both in its living state and 

 when treated with 50 per cent, acetic acid ; the latter reagent 

 causes the thread to become sharply defined, though in the 

 living object it is only to be detected with difficulty (as the 

 tissue dies away it becomes more and more distinct). The 

 young cells pass from the pad to the filament in Forskalea, 

 but to the tentacle in Carinarina. The same is doubtless 

 true for the cnidoblasts of the peronia of the Narcomedusie 

 and of the mantle-rivets which are found in the Geryonidas. 

 Thus the presence of mantle-rivets on the sensory bodies also 

 probably points to the fact that the latter represent rudimen- 

 tary tentacles. 



In conclusion, I would just briefly mention that in tiie 

 investigation of Alcyoniiim acaule I arrived at definite views 

 with regard to the formation of the spicula. In the ecto- 

 derm of this form cells occur to which the term indiffvireut 

 may be applied ; these coalesce here and there into groups, 

 and by fusion give rise to structures which are to be con- 

 sidered as matrix-elements of the spicula. They furnish the 

 form of the future spiculum, and then secrete within them- 

 selves the calcareous substance, in which the nuclei are at 

 first still distinguishable, but which finally so completely fills 

 the whole that nothing more is to be seen of the organic base 

 and the structure appears homogeneous and shining. This 

 transformation is accomplished in the mesoderm. 



* 0. and R. Hertwig, toe. cit. 

 Ann. d: Mag. N. Hist. Ser. 6. Vol. ix. 19 



