ECHINODERMATA 865 
nervous masses, and to support the organs of motion, but in the simple struc- 
ture of the lower animals,-the frame work serves only the last of the pur- 
poses, being either external to the animal substances, as in the Tudipore and 
Sponges, or internal, as in the Sertularie, Gorgonia, &c. The animals of this 
division have been arranged in five classes, viz. 
I. EcurnoperMATA, or animals with a crustaceous covering, distinct intes- 
tinal canal, and organs for generation, respiration, and partial circulation. 
II. Enrozoa, or intestinal worms ; elongated and depressed animals, which 
have no organs for respiration or circulation. 
III. Acarepua. Animals of a circular and radiated form, and destitute 
of circulating and respiratory organs. 
IV. Potyrt, or Zoophytes; comprehending all those small, gelatinous, 
and compound or aggregated animals which have a mouth surrounded by 
tentacula, and conducting into a simple stomach. 
V. The Inrusoria, or those smaller beings only known through the me- 
dium of the microscope, which are found in stagnant waters. The greater 
part of these have a gelatinous body, and are destitute of viscera, though 
some of the species possess visible organs of movement, and a stomach. 
CLASS X.—ECHINODERMATA. 
Body suborbicular, with a coriaceous or crustaceous covering, radiated, desti- 
tute of head, eyes, and articulated feet ; mouth inferior, simple or multiform ; 
organs of digestion compound ; exterior tubes or pores for respiration. 
Tue animals of this class were arranged by some of the older naturalists 
among the testaceous Mollusca; by others among the Zoophytes; while 
others considered them as allied to the Crustacea. The more modern wri- 
ters, however, founding their divisions on the comparative structure of the 
animals, as well as their external characters, have placed the animals of this 
group in a separate class, Cuvier making them the first class of his great 
division of Zoophytes, or animals with prehensile and retractile tentacula, 
and Lamarck placing them also in a separate class, under the title of Radi- 
aria. In this class the radiated structure, both externally and internally, 
forms a distinctive character. The body is generally orbicular, covered. with 
a skin, or a crustaceous or calcareous covering, and often armed with tuber 
cles or jointed and moveable spines. The interior cavity is provided with, 
distinct viscera, and a kind of vascular system maintains a communication. 
with the different parts of the intestine, and with the organs of respiration. 
These organs consist in pores or orifices, or exterior tubes for the passage of 
the water, The animals of this class are destitute of head, eyes, and arti- 
109 73 
