62 ADAM SEDGWICK. 



It is worth while trying to picture how such pores may 

 have arisen. In the supposed ancestor the two layers of 

 the body wall were in more or less close apposition. The 

 animal had no vascular system, and only one more or 

 less differentiated opening, the primitive mouth. It would 

 obviously be convenient that the excretory products should 

 pass out as near as possible to the point where they were 

 formed, or that there should be some arrangement of ducts by 

 which they could be carried to the mouth opening. The latter 

 arrangement does not appear ever to have been developed in 

 the Coelenterata, while the former arrangement is present, if 

 not in all, still in a great number of Medusae and Actinozoa. 

 My knowledge of the physiology of these low animals is not 

 sufficient to enable me to offer any hypothesis of how the pores 

 arose. But I may suggest that in the first instance the endo- 

 derm cells were of one kind only, whose function was to eat 

 (in an amoeboid manner) the food swept into the body cavity 

 through the mouth opening, and to prepare soluble nutritive 

 juices which passed to the ectoderm. The excretion of 

 nitrogenous waste products must have been carried on by all 

 the cells of the body, inasmuch as there is no circulatory 

 system. The immediate undigestible remains or solid excreta 

 from the endoderm cells would be cast into the alimentary 

 cavity. Originally the latter must have been swept to the 

 mouth and so got rid of. As the animal enlarged in size, and 

 no well-developed canal apparatus appeared by which these 

 solid waste products of the alimentary cavity would be directly 

 carried to the mouth opening, some of the endoderm cells at 

 the periphery of the animal became specially modified to eat 

 these products, and pass them through or between the ecto- 

 derm cells to the exterior. So a close connection became 

 established between the cells of the ectoderm and the endoderm, 

 which eventually led to the establishment of a pore, the 

 excretory pores. For. an example of this kind of excretion 

 through the ectoderm, I may refer to Eisig^s ^ observations on 



' " Die Segmentalorgane d. Capitelliden," ' Mitth. a. d. Zool. Stat. z. 

 ;tJeapel,' vol. i. 



