650 ACALEPHA. 



THE THIRD CLASS OF THE RADIATA,— 



THE ACALEPHA,— 



Includes all those Radiated Animals which swim in the waters of the ocean ; and in 

 which we can still perceive vessels, though these vessels are, in truth, little else than 

 intestinal tubes, ramified through the parenchyma of the body. They admit of a natural 

 division into two orders, — Simple and Hydrostatic. 



THE FIRST ORDER OF THE ACALEPHA. 



THE ACALEPHA SIMPLICIA, 



These float and swim in the water, by alternate contractions and dilatations of the body, 

 although their substance is merely gelatinous, and without any apparent fibres. The apparent 

 vessels found in some of them are only hollows in the gelatinous substance originating from the 

 stomach, and oS"ering no proof of a true circulation. There are obvious points of resemblance 

 among them all ; but still tliey admit of division into genera and subgenera. 



Medusa, — 

 Have a central disc, more or less convex, on the upper surface, something like the head of a mushroom, 

 and termed the umbrella. The contractions and dilatations of this disc con- 

 tribute to the locomotion of the animal ; [but they are not powerful enough for 

 stemming rapid currents of the water.] The margins of the umbrella, and those 

 of the mouth, or of the suckers which supply the place of a mouth, in the middle 

 of the under surface of the disc, are furnished with tentacula, very much varied 

 in form and size, and these variations are the basis of many subdivisions of the 

 genus. [They are very numerous ; and the small ones give the seas in which they 

 Fig. 141.— Medusa. abouud the appearance of being crowded with flakes of half-melted snow. Some 

 of these show fine prismatic colours ; and in not a few the gelatinous matter which fills the integument 

 of the disc is of so acrid a nature as to irritate and blister the skin, even after it has been dried.] 



Medusa, properly so called, includes all those that have a true mouth on the under side of the disc ; but this 

 mouth is sometimes a simple opening-, and at other times placed on a peduncle. 



JEquorea, includes those in which the mouth is simple, and not on a peduncle, or furnished with arms or ten- 

 tacula. When there are no tentacula round the disc, they form the Phorcynia of Lamarck. When the disc is 

 furnished with tentacula all round, they are the ^quorea strictly so called, and one of the most numerous in the 

 warm seas. Some have the under surface covered with laminae, and others have the margins of the umbrella 

 diversified by furrows. 



Pelagia, comprehends those which have the mantle produced into a peduncle, or divided into arms or ten- 

 tacula. 



In all these subgenera, there are no lateral cavities ; but in the majority of those with a simple mouth, there are, 

 in the substance of the umbrella, four organs inclosed in furrowed membranes, which, at certain seasons of the 

 year, are tinged with a dark-coloured substance, understood to be the germs of the young. They are lodged in four 

 cavities, which open near the mouth, or the sides of the peduncle ; and as small animals are sometimes entangled 

 in them, some have regarded them as mouths, and others as organs of respiration. That they are not mouths is 

 evident, and the respiration appears to be performed by the margin of the umbrella. The tentacula, whether on 

 the margin of the umbrella, or round the mouth of the animal, vary not only in different species, but in the 

 different ages of the same species. 



Cyanea, — 

 Includes all the species which have a central mouth, and four lateral ovaries. 



C. mirita, is one of the most common and widely distributed species. With age, it acquires four very long 

 arms : the margin of the umbrella :." finely ciliated all round ; and within it are observed reddish vessels origin- 



