RADIATA. 83 
joined to the parent, and finally is cut off altogether, and commences an 
independent existence. . 
The Actinia.—The Actinia, or Sea-anemone, may be taken as a type of 
another chief group of Ccelenterata. There are many species of Actinia, 
and some of them are common on the British shores. It is in tropical 
seas, however, that they are most numerous, and attain the greatest size: 
and beauty. An Actinia consists of a cylindrical body, the lower end 
forming a disc by which the animal is generally attached to a rock or 
other object, whilst at the other end is the mouth surrounded by numerous 
tentacles arranged in circles, like the petals of a flower. Many of the Actinic 
vie with the finest flowers in the delicacy and brilliancy of their colours. 
When left dry by the receding tide, or when rudely touched, they contract 
into a jelly-like mass. In this condition, the most common British species 
must be familiar to every one who has gathered periwinkles, or searched 
for shells on a rocky shore. It is far from being the most beautiful of our 
British Actiniz, but it is,one of those most easily kept in the aquarium. 
It is capable of subsisting for a considerable time without supplies of food, 
but is very voracious, and readily accepts morsels of almost any animal 
substance. When food comes within reach of the tentacles, those nearest 
to it lay hold of it, and convey.it not only into the mouth, but into the 
stomach, not letting go till it is fairly lodged there. Many animals 
seem to have an instinctive horror of the tentacles of the sea-anemone. 
The hermit crab instantly flees out of its shell, if the shell is caught by 
them; as any one may readily observe for himself, who, having found a 
sea-anemone of good size in a rock-pool, brings a hermit crab cautiously 
near to it. 
Radiata. 
The characters upon which Cuvier founded the primary division of 
the animal kingdom which he called Radiata, or Radiated Animals, may 
be seen in their greatest perfection in star-fishes. These creatures belong 
to a class to which the name of Echinodermata! is given from their 
generally rough or spiny integument. They are much higher in organisa- 
tion than any of the groups hitherto noticed; they have a distinct digestive 
system, and a distinct vascular system, although for the former many of 
them have only a single orifice. A circular and radiating nervous system 
has been observed in many. They are especially distinguished from all 
the Protozoa and Celenterata by their well-organised skin, which is often 
strengthened by calcareous plates, and sometimes has the additional pro- 
tection of numerous long spines, as in Sea-urchins; and are further 
characterised by their water-vascular system, or apparatus for water- 
circulation, a peculiarity of the radiate animals. This water-vascular 
1 From Greek echinos, a hedgehog, and derma, skin. 
