12 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



is developed ; that the embryo consists of three germinal layers (external 

 ectoderm or epiblast, internal endoderm or hypoblast lining the gut, 

 and a medium mesoderm or mesoblast lining the body cavity). In the 

 next two classes (Coelentera and Sponges) the conditions are different, as 

 may be expressed in the following table, though it is open to question 

 whether the contrast is quite so great as it seems : 



SI-ONGES AND CCELENTERA. 



HIGHER ANIMALS (GKLOMATA). 



There is no body cavity. There is but 

 one cavity, that of the food canal. 



There is no definite middle layer of 

 cells (mesoderm), but rather a middle 

 jelly (mesogloea). The embryo is 

 diploblastic. 



The radial symmetry of the gastrula 

 embryo is usually retained in the 

 adult, and the longitudinal (oral- 

 aboral) axis of the adult corresponds 

 to the long axis of the gastrula. 



There is a body cavity or coelom be- 

 tween the food canal and the walls 

 of the body. But this is often in- 

 cipient, or degenerate. 



There is a distinct middle layer of cell^ 

 (mesoderm) between the external 

 ectoderm and the internal endo- 

 derm. The embryo is triploblastic. 



The longitudinal axis of the adult does 

 not correspond to the long axis of 

 the gastrula embryo. The adults are 

 usually bilateral, in some cases asym- 

 metrical, in echinoderma radial. 



Coelentera.- -This series includes jelly-fishes, sea-anemones, 

 corals, zoophytes, and the like, most of which are equipped 



FIG. 1 6. Sea-anemones on back of hermit-crab. 

 After Andres. 



with stinging cells, by means of which they paralyse their 

 prey. All but four or five are marine. The body may 

 be a tubular polyp, or a more or less bell-like " medusoid," 



