BDELLOSTOMA DOMBEYI, LAC. 153 



I have already referred to this matter in connection with the 

 brief description of Bdellostoma's gills, but it is sufficiently 

 important to call for a more detailed account, in which I shall 

 try to bring the branchial apparatus of Bdellostoma into 

 harmony with the primitive condition of the same organs in 

 Branschiostoma. 



Having shown that there is a great degree of variability in 

 some of the organs of Bdellostoma, it will be in place to seek 

 for the conditions which keep up this variability, for the 

 conditions must be present conditions and in no sense past 

 ones. Three conditions suggest themselves as probably con- 

 cerned in keeping up this variability among the Bdellostomas 

 as a group of animals of common descent and as families of 

 individuals living under common conditions in circumscribed 

 areas. These are Geographical Distribution, Panmixia and 

 Hermaproditism. 



The geographical distribution of Bdellostoma cannot be 

 satisfactorily accounted for by assuming several separate crea- 

 tions of these animals, for though so widely distributed on the 

 floor of the Indo-Pacinc oceanic depression, all its morphologi- 

 cal characters and its life habits point to its origin from a single 

 ancestor at some place in the South Pacific ocean. Admitting 

 the common descent of the varieties of Bdellostoma, the 

 phenomena connected with the geographical distribution of 

 animals in general lead us to conclude that Bdellostoma has 

 existed for a very long period of time and that it has been but 

 little modified during this long period. We do not know that 

 it has always been as variable as it is at present ; but if it has 

 been, it is a most remarkable case, since ordinary animals 

 would have become well differentiated into distinct species during 

 the process of dispersion over sucJi a great territory and the 

 consequent operating of more or less changed conditions in 

 different ways upon different parts of the variable organism 

 during the great period of time necessary to accomplish this 

 dispersion. Bdellostoma has been widely distributed, and 

 while the conditions in different parts of the territory which it 

 inhabits can hardly be held to be identical, still the sea bottom 

 is far more stable in its temperature and other features produc- 



