438 CLASS ECHINODERMATA. 
known; but it is only by conjecture that we can judge of the 
functions of their singular organs. 
The Potyrt, which compose the fourth class, are all those 
little gelatinous animals, whose mouth, surrounded with ten- 
tacula, conducts into a stomach, sometimes simple, sometimes 
followed by intestines in the form of vessels. It is in this 
class that those innumerable composite animals are found, 
with a fixed and solid stem, which for a long time were re- 
garded as marine plants. 
It is customary to place subsequently to them the thethyz, 
and sponges, although no polypi have yet been discovered in 
them. 
Finally, the INFUSORIA, or fifth and last class of the 
zoophytes, are those little beings which have been discovered 
only by the microscope, and which swarm in stagnant waters. 
The majority of them present nothing but a gelatinous body, 
without viscera. Nevertheless, we have at their head some 
more complex species, possessing visible organs of motion, 
and a stomach. Of these, too, in all probability, at some 
future day, a separate class will be formed. 
THIRD CLASS OF THE ZOOPHYTES. 
THE ECHINODERMATA. 
THE Echinodermata are as yet the most complicated animals 
of this division. Invested with a well organized skin, often 
supported by a sort of skeleton, and armed with points, or 
with articulated and mobile spines, they have an interior 
