18 



THE SUB-KINGDOM CCELENTEEATA. 



by means of a ^'ide inferior aperture {fig, 2, / 



and g). 



Some Adinozoa exhibit a further sub-division 

 of each of the two primary blastodermal layers 

 into other secondary membranes, foreshadowing a 

 structure so constantly met with among the higher 

 animals, just in the same manner as, in a few of 

 the more advanced stomatode Protozoa, an in- 

 distinct differentiation of the body into layers 

 indicates a condition which is manifested, without 

 exception, by the immature forms of the four re- 

 maining sub-kingdoms. 



Development of Animals. 



/^The organism does not ex- A blastoderm is formed, \ 



hibit true layers. wliich divides into inner and 



Photozoa. °^t^^ l^y^^«- 



The alimentary canal freely 

 communioates with the somatic 

 cavity. There is no distinction 

 between neural and hsemal 



regions. 



C(ELENTERATA. 



The two laj-ers of the blas- 

 toderm become further diffe- 

 rentiated. The alimentary 

 canal has no direct communi- 

 cation with the somatic cavity. 

 Distinct neural and hsemal 

 regions appear. 



The htemal region is first 

 developed. The mouth opens 

 on the neural aspect. There 

 is no segmentation of the blas- 

 toderm. 



MOLLUSCA. 



The neural region is first 

 developed. The blastoderm 

 may divide into segments. 



/ The mouth opens on the 

 neural aspect, towards which the 

 limbs are turned. 



ANiatLOSA. 



The mouth opens on the 

 hsemal aspect, towards which 

 the limbs are turned. A pri- 

 mitive groove, dorsal and vis- 

 ceral plates, are formed. 



Vektebeata. 



