248 THE MICROSCOPE. 



fering little, if at all, from the sea-water in which the animal swims; 

 and when this is drained away, so extreme is the tenuity of the 

 membranes which contained it, that the dried residue of a "jelly- 

 fish," weighing only two pounds, which was examined by Professor 

 Owen, weighed only thirty grains. 



The transparency of the tissues renders the whole of the Acalephce 

 delightful objects for the microscope. (See an excellent paper in the 

 Transactions of tJie Microscopical Society, " On the Anatomy of Two 

 Species of Naked-eyed Medusae," by G. Busk, Esq. ; also Professor 

 Forbes' various works on this family.) 



ECHINID^E. 



These creatures form an extensive class, to which the above name 

 has been applied, as well as that of Echino-dermata, Sea-urchins, or 

 Sea-eggs. Asterias, or Star-fish, are the best known family of this class. 



The Echino-dermata are divided into four orders. In two of these 

 the body is more or less flattened or discoid in its form, and usually 

 furnished with five or more arms. These in the first order, the 

 Crinoidea, are slender, and formed of complete calcareous rings or 

 cylinders ; whilst in the second, the Stellerida, the calcareous covering 

 of the arms is composed of separate plates. In the third order, the 

 Echinida, the calcareous plates have become united into a regular 

 shell ; and the fourth includes the worm-like forms, the Holothurice. 



The amount of the calcareous deposit in the skin varies greatly 

 in the different animals composing the class. In some (as the Holo- 

 thurice) it forms small irregular grains, scattered, not very plentifully, 

 through the substance of the skin ; in others, as the Star- fishes (Stel- 

 lerida), it constitutes plates of various forms, fitting closely to one 

 another, but only connected by the agency of the true skin ; so that 

 although the body is completely encased in a suit of calcareous armour, 

 every part of it still retains considerable flexibility. In others again, 

 as the Sea-eggs or Sea-urchins (Echinida), these plates are positively 

 united together, forming a continuous shell, within which all the 

 organs of the animal are enclosed. Upon most of these calcareous 

 plates tubercles are to be seen, which serve for the articulation of 

 movable spines, often of considerable size. These assist the animal 

 in its motions ; and it is from their almost universal presence that 

 the name of the class (Echino-dermata, echinos, a spine, and derma, 

 skin) is derived. 



Echinida may be found in abundance upon our sea-shores, lurk- 



