56 PROFESSOR HUXLEY. 
‘“‘respiratory pore”’ on the left side, during the later periods 
of the tadpole’s life. Here is a structure homologous with 
the splanchnopleure of Amphioxrus ; while, in the thoraco- 
abdominal region, the splanchnopleure appears to arise by 
splitting of the mesoblast. Considering what takes place in 
Amphioxus, the question arises whether the “ splitting” of 
the mesoblast in the Vertebrata may not have a different 
meaning from the apparently similar process in the Arthro- 
poda, Annelida, and Mollusca; and whether the pericar- 
dium, pleura, and peritoneum are not parts of the epiblast, 
as the atrial tunic is of the epiblast of the ascidians. Further 
investigation must determine this point. In the meauwhile, 
on the assumption that the pleuro-peritoneal”’ cavity of the 
Vertebrata is a virtual involution of the epiblast, the peri- 
toneal aperture of fishes becomes truly homelogous with the 
“respiratory pore” of Amphiowus; and the Wolffian ducts 
and their prolongations, with the Miillerian ducts, are, as 
Gegenbaur has already suggested, of the same nature as the 
segmental organs of worms. 
The division of Merazoa without an alimentary cavity is es- 
tablished provisionally, for the Cestoidea and Acanthocephala, 
in which no trace of a digestive cavity has ever been detected. 
It is quite possible that the ordinary view that these are 
Gastreee modified by parasitism is correct. On the other 
hand, the cases of the Nematoid worms and of the 7rematoda 
show that the most complete parasitism does not necessarily 
involve the abortion of the alimentary cavity, and it must be 
admitted to be possible that a primitive Gregarinform para- 
site might become multicellular and might develep repro- 
ductive and other organs, without finding any advantage in 
an alimentary canal. A purely objective classification wil 
recognise both these possibilities and leave the question 
open. 
