ANIMALIA RADIATA. 499 



better known, will perhaps form a separate one; as yet however we 

 only conjecture the functions of their singular organs. 



The POLYPI, which compose the fourth class, are those little ge- 

 latinous animals whose mouth surrounded with tentacula leads to a 

 stomach sometimes simple and sometimes followed by intestines in 

 the form of vessels. To this class belong those innumerable com- 

 pound animals with a fixed and solid stem which were considered as 

 marine plants. 



The Thethyiaa and Sponges are usually placed at the end of this 

 class, although Polypi have not yet been discovered in them. 



The INFUSORIA, or the fifth and last class of the Zoophytes, are 

 those minute beings whose existence we have only discovered by 

 means of the microscope, and which swarm in stagnant waters. 

 Most of them have merely a gelatinous body destitute of viscera, al- 

 though we commence the series with more compound species pos- 

 sessed of visible organs of locomotion and a stomach: these also 

 may hereafter constitute a separate class. 



CLASS I. 



ECHINODERMATA.(l) 



The Echinodermata are the most complicated animals of this di- 

 vision. Invested with a well organized skin, frequently supported by 

 a sort of skeleton, and armed with points, or movable and articulated 

 spines, they have an internal cavity in which distinct and floating 

 viscera may be perceived. A sort of vascular system, which it is 

 true does not extend throughout the body, keeps up a communica- 

 tion with various parts of the Intestine, and with the organs of res- 

 piration, which are generally very distinct. Threads are also seen 

 in several, which may act as nerves, but which are never arranged 

 with the regularity and fixed order of those in the animals of the 

 two preceding divisions of the Invertebrata. 



_ - _ 

 (I) Hedge-hog (i. e. spiny) skinned, 



