STRUCTURE OF RADIATA. ECHINODERMATA. 125 



117. The circular arrangement of the organs of Eadiated 

 animals is a striking point of resemblance to the Vegetable 

 kingdom ; and it has frequently caused mistakes to be made 

 in regard to the Sea- Anemones and other large polypes, 

 which, when their mouths are open and their arms spread 

 out, look so much like the blossoms of some of the Com- 

 posite tribe of plants, as to have received the name of animal 

 flowers. But there is yet a stronger analogy between the 

 lower members of the Eadiated group and the Vegetable 

 kingdom; for among the former, as in the latter, we find 

 a union of many individuals, which are capable of existing 

 separately, into one compound structure, having a plant-like 

 form. This is the nature of the stem of Coral (fig. 76); 

 which is, in fact, the skeleton of one of these compound 

 systems, consisting of a number of polypes united by a jelly- 

 like flesh ; just as the woody stem of a tree is the skeleton 

 that supports a vast number of buds, each of which is capable 

 of living by itself. This aggregation results from the in- 

 definite multiplication of parts by the process of gemmation 

 or budding, and from the persistence of the connexion 

 between these parts, notwithstanding that, if separated, 

 they can maintain an independent existence. To the tree- 

 like fabrics thus produced, the name Zoophytes (animal 

 plants) is commonly given; and ordinary observers often 

 find it difficult to get rid of the idea of their vegetable 

 origin. The animals that formed them are, of course, fixed 

 to one spot during all but the earliest periods of life ; and 

 the amount of movement which they perform, for the pur- 

 pose of obtaining and securing their food, is very little 

 greater than that which is witnessed in the Sensitive plant 

 and Venus's fly-trap. 



118. The class of ECHINODERMATA receives its name from 

 the prickly character of its covering, which is evident enough 

 in the Echinus or Sea- Urchin, and in the Star-fish; but there 

 are other animals, sufficiently resembling these in general 

 structure to be united in the same class, which have a body 

 entirely soft, namely, the Holothurice (fig. 67), commonly 

 termed Sea-Cucumbers. This class ranks as the highest 

 among the Eadiata, in regard to general complexity of struc- 

 ture. The skeleton of the Sea-Urchin, Star-fish, and other 

 animals resembling them, is a box-like shell or " test," formed 



