ECHINODERMATA 



167 



stars and sea lilies differ from most 

 animals in being attached to some 

 object by a stalk. They usually 

 live at considerable depth in the 

 sea. Fossil remains show that these 

 animals were very abundant several 

 millions of years ago in the- Silurian 

 period. To-day but few species exist. 

 The Echinodermata are lower in the 

 animal scale than most of the preceding 

 phyla, as they have no heart and very 

 simple nervous and excretory systems. 

 The senses of sight and hearing seem to be 

 entirely wanting, and the sense of smell 

 is very little developed. All the species 

 live in the same environment and present 

 the same characteristic radiate structure. 



FIG. 193. Crinoid show- 

 ing the stem with which 

 it is attached. 



13. CCELENTERATA AND PORIFERA 



The two groups of animals discussed in this chapter were 

 formerly placed in the same phylum because they are so 



much alike in hab- 

 its and structure. 

 The stinging or 

 nettle cells pos- 

 sessed by the Coelen- 

 terata and wanting 

 in the Porifera 

 separate clearly 



. 194. Diagrammatic cross section of the body of a 

 coelenterate A, and that of a worm or insect B. the tWO groups. 



