KEY TO CLASSES AND ORDERS OF ANIMALS il 



Class CALCAREA 



1(2) Body wall thin and porous, central cavity lined with 

 collar cells; Ascon t3^e. (Pratt, 75.) 



Order Homocoela 



2(1) Body wall thicker, central cavity without collar cells; 

 Sycon t5^e. (Pratt, 76.) Order Heterocoela 



Class DEMOSPONGIAE 



1(4) Skeleton containing silicious spicules with or without 

 homy substance (spongin). 2 



2(3) Four-rayed silicious spicules present; spongin and 

 needle-like spicules absent. (Pratt, 78.) 



Order Tetractinellida 



3(2) Needle-like spicides present; spongin fibers present or 

 absent; some in fresh water. (Pratt, 80; W. and W., 

 306.) Order Monactinellida 



4(1) Skeleton lacking silicious spicules. 5 



5(6) Skeleton of spongin, marine. The bath sponges. 

 (Pratt, 84.) Order Ceraospongiae 



6(7) Skeleton wanting. (Pratt, 85.) Order Myxospongiae 



Phylum COELENTERATA 



I (4) Small hydroid polyps with either small or large medusae 

 or none. 2 



2(3) Hydroid form when present without gastric ridges or 

 filaments. Reproductive organs almost always ecto- 

 dermal. Medusa, when present, with a velum. 

 Hydroid frequently branching. Some fresh-water 

 forms. Class Hydrozoa, p. 12 



3(2) Hydroid small, if present, and bears gastric ridges. 

 Medusa almost always present but without a velum; 

 marine. (Pratt, 126.) Class Scyphozoa 



