XIV INTRODUCTION. 



animal nature of the sponges there are sufficient 

 grounds for considering them animals and as 

 belonging to this class, and they are omitted 

 here only from a desire of further investigation. 



The British polypous Zoophytes naturally 

 divide themselves into two grand divisions or 

 sub-classes ; the first embraces the radiated 

 form, in which the body is contractile in every 

 part with but a single aperture. This division 

 contains three minor ones, or orders, the Hy- 

 dro ida, Asieroida, and Helianthoida. The se- 

 cond division contains the Molluscan Zoophytes, 

 in which the body is non-contractile, non-sym- 

 metrical, with two apertures. It has only one 

 order, the Ascidioida which is very closely 

 allied to the Mollusca tunicata. In the Hydi*oida 

 the tentacula are tuberculated, and the stomach 

 a mere depression made in the granular mass 

 without any particular organization, and the 

 horny sheath which is not found in all the genera, 

 is external. In the Asteroida, the tentacula are 

 eight in number and fringed ; the solid parts 

 when present, are all internal; and the external 

 surface is always marked with eight rayed de- 

 pressions. The Helianthoid having for its 

 type the common Anemone, need hardly be re- 

 fered to now. In the second sub-class there is 

 a considerable advance in the complication of 

 the polype. In the orders of the first division, 

 the tentacula are prehensile and used only for 

 capturing prey; in those of the second, they are 

 clothed with vibratile cilia and are chiefly sub- 

 servient to respiration. In the first three orders, 

 the polypes are comparatively sluggish, in the 

 last they are remarkable for the rapidity of their 

 actions. 



Though these productions are low in the scale 

 of being, and the polypes for the most part 



