15 ZOOPHYTES. 



10. General Divisions of Zoophytes. Zoophytes constitute natu- 

 rally two distinct groups, differing in mode of reproduction and in 

 internal structure. The visceral cavity in some of them is, as de- 

 scribed, a simple tubular sac. In others, it is divided vertically by 

 fleshy lamellae, proceeding from the walls and forming a radiate series 

 around the cavity. Connected with these peculiarities, we observe 

 striking differences in the mode of ejecting the ovules. When there 

 are lamellae in the cavity, the ovules are formed by them, and appear 

 in clusters attached to the margin, from which they are finally de- 

 tached, and make their escape out of the mouth. But when there 

 are no lamellae, the ovules are produced in the walls of the visceral 

 cavity, and make their readiest escape outward through the sides of 

 the polyp, instead of the more indirect route, into the visceral cavity 

 and out of the mouth. 



The following are the divisions based on the characters mentioned :* 



'o 



Order Hydroidea. 



Visceral cavity, a simple tubular sac ; reproductive functions re- 

 siding only in the walls of tlie cavity ; young or ovules pullulating 

 from the sides of the parent. 



Order Actinoidea. 



Visceral cavity, divided vertically by fleshy lamellae, which possess 

 reproductive functions; ovules formed within the cavity from some of 

 the lamellae and ejected through the mouth. 



The polyps of the order Hydroidea are mostly minute, and the 

 coralla, when any are formed, are either horny or membranous ; they 

 are very delicate, and, when compound, usually consist of minute 

 calictesf (or little cups), arranged in series along a tubular axis. 



tentacles are furnished with vibralile cilia. Excepting their calcareous secretions and 

 mode of budding, they are widely removed from true zoophytes. (See farther §§ 106-8.) 



The definition also excludes sponges, as already intimated, which, excepting the most 

 general attribute of animal matter (if they are animals), possess nothing in common with 

 the polyp. No single character, except their forms, has ever been pointed out which 

 indicates a relationship. 



* The names below have the following derivations : — Hydroidea is from the included 

 genus Hydra, and Actinoidea from the included genus Actinia. This last name (from 

 axTiv, ray of the sun,) alludes to the radiated character of the animals. 



■f The term calicle (from caliculus, a little cup,) is used for the prominences which con- 

 tain the cells in many corals ; and cell is restricted to the cavity itself. By cellide, as here- 

 after used, the minute pores of the corallum will be referred to. 



