CHAPTER V, 



THE BRANCH OF RADIATA, OR RADIATES. 



THIS Branch includes all animals whose parts radiate 

 from a vertical axis ; or, in other words, whose structure 

 clearly exhibits the idea of radiation. The Radiates are 

 all aquatic, mainly marine, and constitute the lowest 

 Branch of the Animal Kingdom. There are at least ten 

 thousand living species distributed among three classes, 

 Echinodermata or Echinoderms, Acalephs or Jelly- 

 Fishes, and Polypi or Polyps. 



SECTION I. 



THE CLASS OF ECHINODERMATA, OR ECHINODERMS. 



THIS Class comprises radiates which have a tough cov- 

 ering containing more or less calcareous particles, or a 

 shell composed of pieces which are movable, or bound 

 together and covered with tubercles or spines, the body 

 divided into two well-marked regions, the oral and ab- 

 oral, or actinal and abactinal,* and all the parts radi- 

 ating from the oral opening and meeting in the ab-oral 

 region, and along certain of the rays regular rows of tubu- 

 lar suckers used in locomotion, and called ambulacra ; and 



* Professor Agassiz proposes the following new names for the different 

 parts of Radiates, viz.: actinostonie for the so-called mouth; spherosomt 

 for the body wall ; actinal for the pole where the actinostome is situated ; 

 abactinal for the opposite pole ; and spheromeres for the homological seg- 

 ments of the body. 



