128 LIFE AND DEATH. 



escaped, the thin disintegrated membrane, composed of ciliated 

 cells, is no longer in a condition to live, and dies at once. This is 

 the course of events as described by Gotte, and he is probably 

 correct in his interpretation. This is the real death of the Ortho- 

 nectides, and if we regard them as low primitive forms (Mesozoa), 

 here for the first time in the ascending- series we meet with natural 

 death. But the causes of this are scarcely so clear as Gotte seems 

 to think when he ascribes it to the effect of reproduction an 

 effect which is ' not only empirically necessary, but absolutely 

 unavoidable.' Such a necessity is explained by the fact that the 

 endoderm consists entirely of germ-cells. Now the life of the 

 organism, being dependent upon the mutual action of both layers, 

 must cease as soon as the whole endoderm is extruded during- repro- 

 duction. 



Arguments such as these pass over the presence of a mesoderm ; 

 but apart from this omission, it does not appear to me so self- 

 evident from a purely physiological standpoint, that the ectodermal 

 sheath with its muscle layer must die after the extrusion of the 

 g-erm-cells. 



In those females to which Gotte refers in this passage, the whole 

 sheath remains at first quite uninjured, with the exception of a 

 small cap at the anterior end, which is pushed off to give exit to 

 the ova ; and inasmuch as the sheath continues to swim about in 

 the nutritive fluids after this has taken place, the proof is at any 

 rate wanting that it cannot support itself quite as well as before, 

 although it has lost the germ-cells. 



Then why does it die ? Mv answer to this is simple : because 

 it has lived its time ; because its length of life is limited to a 

 period which corresponds with the time necessary for complete 

 reproduction. The physical constitution of the body is so regulated 

 that it remains capable of living until the extrusion of the repro- 

 ductive cells, and then dies, however favourable external conditions 

 may be for its further support. 



The correctness of this explanation is shown by a consideration 

 of the males and the second form of females ; for in these cases the 

 body falls to pieces, not as a consequence of reproduction, but as a 

 preparation for it ! 



Gotte only mentions the second female form in a note, in which 

 he says, it appears ' that in the second female form of these animals 



