ingestive apertures being developed in the lateral walls of the gas- 



Monostomata, that is to say, the gastrula develops but one' inges- 

 gastrulee are produced by germination is of course not a real ex- 



This division includes two groups, the members of each of which 



morpha. Under the latter head are included the Turbellaria, the 

 Nematoidea, the Trematoda, the Hirudinea, the Oligochaeta, and 

 probably the Rotifera and Gephyrea. 



In all the other Monostomata the primitive opening of the gas- 

 trula, whatever its fate, does not become the mouth, but the latter 



these Deuterostomata there is a perivisceral cavity distinct from 

 the alimentarv canal, but this perivisceral cavitv is produced in 

 different ways. 



1. A perivisceral cavity is formed by diverticula of the ali- 

 shown that, not only the ambulacral vessels, but the perivisceral 



forgotten that 

 la are truly » cce 

 nth the t'uudamei 



2. A perivisceral cavitv is tunned by the splitting of tl 

 blast (Schizocoela). 



This appears to be the case in all ordinary Mollusca, i 

 polychetons Annelida, of which the Mollusca are little m. 

 an oligomerous modification, and in all the Arthropoda. 



It remains to be seen whether the Brachiopoda and the 

 belong to this or the preceding division. 



