AQUATIC ORDERS. 141 



Nudibranchiata, (literally, having the branchise naked.) 

 In these animals the branchise form appendages, on the 

 external surface of the body above. Sometimes they 

 extend along the whole of the back, and at others they 

 are confined to the posterior extremity of the dorsal 

 surface, as in the doris, where they form a radiating 

 flower, each petal having a beautifully fringed, or rather 

 arborescent, margin. 



Closely allied to this order is the next, termed Infero- 

 bi'anchiata, (having the branchise below the mantle.) 

 In the animals of this group the edge of the mantle 

 covers on each side a row of leaf-like appendages, which 

 are the organs of aquatic respiration. 



In the TedibrancJdata (having concealed branchiae) 

 the respiratory appendages form a fringe along one side 

 of the body only, buried in a deep furrow between the 

 mantle and the ventral disc or foot : of these the pleuro- 

 branchus and the bulla are examples. 



In the Heteropoda (so called because the foot, instead 

 of forming a disc upon which to creep, is modified into 

 a vertical muscular swimming apparatus) the branchiae 

 constitute plume-like appendages seated on the posterior 

 part of the back, and directed forwards. The body is 

 of a gelatinous transparent composition, and in swim- 

 ming, the back is downwards, the swimming organ 



