Chap. III.] CCELENTERATA. 37 



organisms ; differentiation will lead to greater sub- 

 division of labour, and greater complexity of struc- 

 ture, but all the materials are, even so low in tlie 

 grade of animal life, ready to our hand. 



"Fig. 9. Perigonimus vestitus, showing Trophosomes and Gonosoines 

 (After Allman.) 



A ccelenterate animal, then, is one in which the 

 archenteron of the gastrula, even when secondary 

 outgrowths are developed from it, remains always as 

 the only cavity in the body, in which the mesoblast 

 is but imperfectly differentiated, but in which organs 

 of offence, locomotion, and sensation are added on to 

 the structures of the original gastrula form. 



