CHAPTER V. 



THE BRANCH OF RADIATA OR RADIATES. 



SECTION I. 



THE RADIATA CONSIDERED AS A BRANCH. 



IN all the animals of the preceding branches we have 

 found an anterior end and a posterior end, a right side and 

 a left side, an upper side and a lower side, more or less 

 plainly marked. 



But the Radiates are so constructed that, apparently, 

 they have no forward end, no posterior end, no right side 

 no left side ; but all their parts radiate from a center, or, 

 rather, from a central axis. In a word, they are animals 

 built essentially on the vegetable type of structure.* 



All radiates are aquatic, and nearly all marine. They 

 comprise three Classes : 



1. ECHINODERMATA or radiates with a tough skin, containing 



calcareous particles ; or with a shell composed of calcareous 

 plates, movable or fixed, and bearing tubercles or spines. Sea- 

 cucumbers, Sea-urchins, Star-fishes, Serpent-stare, and Cri- 

 noids are examples. 



2. Ac ALEPH^E or radiates of a jelly-like consistency ; as Jellyfishes. 



3. POLYPI or radiates which are flower-like in form, and with the 



body divided into vertical chambers by vertical plates ; as 

 Coral-animals and Sea-anemones. 



Some naturalists refer the forms here called Radiata to 

 two sub-kingdoms Echinodermata and Coalenterata. 

 (See foot-note, p. 284.) 



* The above is true as a general statement. But it must be added here that most 

 radiates exhibit some indications of an anterior and a posterior end, and more or less 

 of bilateral symmetry. 



It may be added here that Von Baer has shown that in each of the four great 

 branches of the Animal Kingdom there is a special mode of development in the egg. 

 In the Vertebrates the germ divides into two folds one turning upward, and the 

 other downward. In the Articulates, the germ lies with its back on the yolk. In 

 the Mollusks the germ lies upon the yolk and absorbs it into the under surface of the 

 body. And in the Radiates the germ occupies the whole periphery of the sphere. 



