68 RADIATED AND ARTICULATED TYPES. 



Radiata are fixed to one spot during all but the earliest period 

 of their lives ; and though the instruments by which they obtair 

 their food have some power of spontaneous motion, and certain 

 of their actions appear to indicate sensation, yet it cannot be 

 doubted by any one who observes their habits, that they possess 

 these endowments in a very feeble degree. The most active 

 among them are some of the Medusce, or Jelly-fish tribe, which 

 exhibit the greatest departure from the general type of the 

 Radiata, in the bi-lateral symmetry of their bodies; and certain of 

 the Holothuriw, or Sea-Cucumbers (Fig. 28), in which the body 



IMG. 28. HOLOTUURIA. 



is so prolonged, as to remind us of that of a Leech or Caterpillar; 

 although the circular arrangement of the organs around the 

 mouth shows their place to be in the Radiated sub-kingdom. 



47. In the sub- kingdom Articulata, there is also a consider- 

 able repetition of parts ; which, in the lowest of the group, are 

 almost precisely similar to each other. These parts are disposed, 

 however, not around a centre, but in a line, as in the Centi- 

 pede (Fig. 29) ; and there is a most exact similarity between the 

 two halves of the body, or, in other words, a most complete 

 bi-lateral symmetry. There is no internal skeleton ; but the 

 hard envelope gives the required degree of protection, and also 

 furnishes points of attachment to the muscles by which the 

 body is moved. Where there are distinct members or limbs for 

 aiding the motions of the fabric, we find that they too are covered 



