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 TuUpora and 

 Sponges, or internal, as in the Scrtidarice, Gorgonia, &c. The animals of this 

 division have been arranged in five classes, viz. 



I. EcHiNODERMATA, or animals with a crustaceous covering, distinct intes- 

 tinal canal, and organs for generation, respiration, and partial circulation. 



II. Entozoa, or intestinal worms ; elongated and depressed animals, which 

 have no organs for respiration or circulation. 



III. AcALEPHA. Animals of a circular and radiated form, and destitute 

 of circulating and respiratory organs. 



IV. Polypi, or Zoophytes ; comprehending all those small, gelatinou:^, 

 and compound or aggregated animals which have a mouth surrounded by 

 tentacula, and conducting into a simple stomach. 



V. The Infusoria, or those smaller beings only known through the me- 

 dium of the microscope, which are found in stagnant waters. Tiie 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. — E CHINODERM AT A. 



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. 



The 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 



