Martin, A Note on the Occurrence of Nematocysts and Similar Structures etc. 269 



atteint line longueur de 0.1 mm (PL IX, fig. 1). C'est aux cellules 

 urticantes des tentacules de Cydippe, que nos cellules ressemblent 

 le plus." 



In this instance I am inclined to regard these structures until 

 further evidence is forthcoming as Pseudocnidae. 



III. Conclusions. 



If all the instances of the occurrence of Nematocysts in various 

 animals mentioned above were true Nematocysts developed by their 

 possessor the presence of these structures in such isolated instances 

 in the various groups would, I believe, present great difficulties 

 to an evolutionist. It seems to me that if this view were accepted 

 there would be only two possible explanations to be offered 



(1) that these structures are homologous, i. e.. they have been 

 derived from Coelenterate ancestors possessing these structures 



(2) that these structures are analogous and have been evolved 

 afresh in each instance. 



The first explanation would appear to lead at once to insuper- 

 able difficulties. As I have already stated in my earlier paper on 

 this subject it would involve the acceptance of the view that the 

 Microstomum which contained Cordylophoran Nematocysts must be 

 traced back to a Cordylophoran line of ancestry, w r hereas the com- 

 moner type of Microstomum with Hydra Nematocysts must be 

 traced back to a Coelenterate of the Hydra type with its three 

 distinct forms of Nematocysts. In the case of the Aeolids the 

 acceptance of this view would lead to even more startling results. 

 If on the other hand the Nematocysts in all these instances are 

 explained as analogous structures due to convergent evolution, the 

 isolated appearance of these structures in one or two species in 

 each group would again appear to strain this hypothesis to the 

 breaking point, and it appears to me that even without the experi- 

 mental evidence given above as to the origin of these structures 

 in some animals, the conception of Cleptocnids must have forced 

 itself on the mind of the observer as the only explanation of the 

 curious distribution of these structures in the animal kingdom. On 

 the other hand it must be admitted that the polar capsules of 

 Sporozoa, the Pseudocnidae of the two species of Nemertinea and 

 possibly of the Turbellarian Ofopfftna setosa and the true Nemato- 

 cysts of the Coelenterates afford a most amazing instance of what 

 is apparently convergent evolution. In these cases in which appar- 

 ently all possibility of an explanation based on homology is ex- 

 cluded, we find different animals all of which have developed a 

 similar though not identical complicated mechanical structure. 



In conclusion I should like to draw attention to a curious 

 feature in the behaviour of wandering cells carrying Nematocysts. 

 XXXIV. is 



