RADIATA. 171 



which there is no right and left, no anterior and poste- 

 rior extremity, no above and below. It is true that in 

 some of them there are indications of that bilateral sym- 

 metry which becomes a law in the higher animals ; but 

 whenever such a tendency is perceptible in the Radiates 

 it is subordinate to the typical plan on which the whole 

 group is founded." * 



3. Radiate animals are subdivided into I. CcELEN- 

 TERATA, or Ccelenterates, (koilos, hollow ; enteron, intes- 

 tine,) or animals with an alimentary canal communicating 

 with the general cavity of the body ; and, II. ECHINO- 

 DERMATA, (echinos, a spine ; derma, skin,) or spiny- 

 skinned animals. Other characteristics, however, besides 

 those signified in the names of these sub-types are nec- 

 essary to. be considered. 



4. The CCELENTERATA are radiate animals with a dis- 

 tinct body-cavity, whose walls consist of two layers of 

 cellular tissue, an outer (ectoderm) and inner, (endodenn,) 

 and contain nettling thread-cells, or small sacs full of 

 fluid connected with barbed filaments, capable of being 

 projected for stinging purposes. Most of these animals 

 have hollow tentacles round the mouth. There are two 

 large classes of Ccelenterates : I. The HYDROZOA, which 

 have no digestive cavity separate from the rest of the 

 mass which forms the body, and whose reproductive or- 

 gans are external ; and, II. ACTINOZOA, which have a 

 digestive canal distinct from the rest of the body, sus- 

 pended by radiating partitions, called mesenteries ; and 

 whose organs of reproduction are internal, placed on the 

 mesenteries. The first of these classes may be repre- 



* Agassiz. 



