PEDICELLATA. 639 



matter. The Thethya and Sponges nave been joined to this class, though their 

 animals have not yet been observed. 



5. Infusoria [Animalculse] , the most minute members of the Animal Kingdom, and 

 for the most part microscopic. Some have a very complicated organization, and some 

 appear to be mere particles of animated jelly. [They exist in countless myriads, 

 principally in stagnant water, and some are so tenacious of life, that, after having 

 been for some time dried to powder, they revive again when moistened.] 



THE FIRST CLASS OF THE RADIATA. 

 THE ECHINODERMATA. 



These have a well-organised skin ; sometimes a sort of skeleton ; a digestive and a 

 vascular system ; and a sort of radiating nerves. There are two orders : those with 

 feet, or vesicular appendages answering the same purpose, and those without. 



THE FIRST ORDER OF THE ECHINODERMATA. 



PEDICELLATA. 



These have the skin pierced with numerous small holes, through which protrude cylindric 

 tentacula, terminating in suckers. These are extended or retracted by a humour distinct from 

 that of the intestines, discernible in some of the species, and answer the purpose of feet, by 

 which they perform their locomotion, or adhere to the rocks. Vessels from these continue to 

 unite in a trunk for each row, which trunk terminates near the mouth. The order consists of 

 three very natural faniiUes. 



THE FIRST FAMILY OF THE PEDICELLATA,— 

 The Asterias [Star-fish], — 

 So called, because the body is generally in the form of a star with five rays. Some, however, as 

 A. discoidea, have the body a pentagon, with straight sides ; others, as A. membranacea, have a re- 

 entering angle in each side ; and others, again, as A. tesselata, have the sides concave. 



The frame-work of the body is composed of horny pieces, variously arranged. In those which have 

 distinct rays, there is a longitudinal groove in the upper surface of each 

 ray, perforated on both sides, for allowing the action of the feet ; and 

 all the surface is covered with pores leading to small tubes which admit 

 water, probably for the purpose of respiration. On the central disc, but 

 toward one side of it, there is a stony plate, and below it a canal filled 

 with calcareous matter ; and it is probable that this is the apparatus by 

 which the hard matter of the body is elaborated. There is a sort of verte- 

 brated osseous column in each ray ; and some of the species have osseous 

 plates, and spines on the sides of the rays. Internally, they have one 

 Fit;. 13s.— Asterias. stomach, with two branched coeca extending to each ray ; each ray, 



also, contains two ovaries, and it is understood that they propagate by self-impregnation. The rays 

 are easily reproduced, for the central disc and one ray will reproduce all the others. The mouth, whicli 

 is the only opening to the alimentary organs, is on the under side of the central disc. According to 

 Tiedemann, the principal nerve surrounds the mouth, and sends off a filament to each arm. Such arc 



