III.] LIFE AND DEATH. 1 43 



of doubtful origin. Both species have furthermore lost much 

 of their original nature, both in structure and function, as a 

 result of their parasitic mode of life. Of the Orthonectides 

 and Dicyemidae we know something, but of the reproduction 

 in the single free non-parasitic form, discovered by F. E. 

 Schulze and named by him Tridwplax adhaerens, we know 

 nothing whatever, and of its vital phenomena too little to be 

 of any value for the purpose of this essay. 



At this point it is advisable to return once more to the 

 derivation of death in the Metazoa from the Orthonectides, as 

 Gotte endeavoured to derive it, when he overlooked the fact 

 that, according to his theory, natural death is inherited from 

 the Monoplastids and cannot therefore have arisen anew in 

 the Polyplastids. According to this theory, death must neces- 

 sarily have appeared in the lowest Metazoa as a result of the 

 extrusion of the germ-cells, and by continual repetition must 

 have become hereditary. We must not however forget that, 

 in this case, the cause of death is exclusively external, by 

 which I mean that the somatic cells which remained after the 

 extrusion of the reproductive cells, were unable to feed an}^ 

 longer or at any rate to an adequate extent ; and that the cause 

 of their death did not lie in their constitution, but in the un- 

 favourable conditions which surrounded them. This is not so 

 much a process of natural death as of artificial death, regularl}'- 

 appearing in each individual at a corresponding period, because, 

 at a certain time of life, the organism becomes influenced by 

 the same unfavourable conditions. It is just as if the conditions 

 of life invariably led to death by starvation at a certain stage in 

 the life of a certain species. But we know that death arises 

 from purely internal causes among the higher Metazoa, and 

 that it is anticipated by the whole organisation as the normal 

 end of life. Hence nothing is gained by this explanation 

 founded on the Orthonectides, and we should have to seek 

 further and in a later stage of the development of the Metazoa 

 for the internal causes of true natural death. 



Another theory might be based upon the supposition that 

 natural death has been derived, in the course of time, from an 

 artificial death which always appeared at the same stage of each 

 individual life— as we have supposed to be the case in the 

 Orthonectides. I cannot agree with this view, because it 



