110 



LIFE, IK ITS LOWER FORMS. 



minute red glands, that are common to the integument 

 of the whole class, and this is covered with a series of 

 vibratile cilia. 



Conjecture has been busy upon the use of these very 

 curious organs in the economy of the animal : but abso- 

 lutely nothing is yet certainly known on the subject. 

 Their prehensile power is obvious : bnt whether this is 

 exercised in the way of defensive weapons, or as hands to 

 catch food and hand it to the mouth, is among the things 

 that we have yet to learn. 



The Sea-urchin maybe considered as the type or model 

 of the radiate structure, to which we have traced the gra- 

 dual approach from the polype-form, through the Feather- 

 star, which, in its pentacrine condition, retains the aspect 

 of a true Polype. But in nature there is a constant pro- 

 gression : and we must now briefly glance at the transition 

 from this perfection of radiism to another sort of symmetry 

 — bilateralism, in which there are parts which we can dis- 

 tinguish as right and left, dorsal and ventral, head and 

 tail. This symmetry now begins to appear, and hence- 

 forward is found to characterise the whole range of animate 

 existence. 



On turning over stones at the lowest verge exposed by 

 the retiring tide, — a means of acquaintance with strange 

 creatures which no marine naturalist neglects whenever he 

 has the opportunity, — we occasionally find adhering to 

 them cylindrical, or rather pentagonal leathery animals, 

 very much resembling small cucumbers. They are the 

 representatives of an extensive family of this class, called 

 Holothuriadce. 



If we place one of these in w T ater we shall see the fol- 



