met with during a Sea Voyage. 599 



often seen drawn close to the body of the animal, and partially 

 dissoved. 



The light blue, purple, and pink tints possessed by the 

 Physalia soon fade after being removed from the water. 



The Medusae are more frequently seen than the Physalia, 

 and are more extensive in their distribution. Masses of these 

 fishes are continually seen lying on our shores, or swimming 

 in our estuaries by the contractions and expansions of their 

 bell-shaped mantle. Their size is very various : generally, 

 those I have seen in the tropics are small, but often they attain 

 a large size. They consist of a mantle, which is hemi- 

 sperical, convex before, concave behind, and having tentacula 

 projecting from the margin of the mantle, and broad pro- 

 cesses from the centre of its concave surface. The commonest 

 form possess four larger leaf-shaped processes, and eight mar- 

 ginal tentacula; and are of a yellowish or brownish colour 

 throughout. On the convex margin of the mantle, there is a 

 central dark spot, from which lines radiate to its circumference : 

 of these I have generally counted about thirty-two ; and they 

 constitute the narrow cords described by Dr. Grant. The 

 digestive cavity is situated within the mantle, and branches 

 towards its circumference : the aperture is situated below, 

 between the roots of the leaf-shaped processes. John Hunter, 

 and, since his time, Cuvier, regarded the digestive canal as 

 acting also the part of a circulating organ ; and it is pro- 

 bable that, besides these functions, the water flowing through 

 it is accessory to the respiration of the animal. When first 

 captured, the motions of these little animals are extremely 

 active, swimming by the contraction and expansion of their 

 mantles ; whirling round, and turning backwards, when they 

 meet with any obstacle. Their size I have stated to vary 

 greatly; and their form and colour are not less subject to 

 variety: this may be in the shape of the mantle, the number 

 of the tentacula, or the number and shape of their leaf-like 

 processes. 



Perhaps, however, the most common forms of Acalepha* 

 are the Vellellae ; these having a cartilaginous dorsal plate, from 

 which projects a vertical one. They are common in our seas, 

 exceedingly so off the Cape of Good Hope, where they form 

 an article of food to the numerous sea-fowl, and, it is said, of 

 whales ; and almost cover the sea in a calm in the tropics. In 

 length, they are 2 or 3 inches, and f in. in breadth ; but often 

 they do not attain this size. Their horizontal plate is of an 

 oblong form, crossed diagonally by the vertical one, which is 

 somewhat semi valvular. The border of the first supports ex- 

 ternally the deep blue mantle, and, below, the tentacula, the 



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