RAYS 57 



almost entirely on the sea bottom. Looked at from this 

 point of view, the changes in the form of the pectoral fins and 

 the position of the external gill-clefts are seen to have a 

 definite meaning. The pectoral fins in the rays have become 

 enlarged (in the more specialized forms enormously so), and 

 have not only extended forward, but have also become joined 

 to a greater or lesser extent to the sides of the body or head. 

 This arrangement, which eventually results in the formation 

 of a flat disc in which the limits of the head, the body, and 

 the pectoral fins are indistinguishable, has led to the restriction 

 in the size of the external gill-clefts, and has also forced them 

 downwards on to the under-side of the fish. Thus, however 

 shark-like in appearance a member of this Order may be, its 

 true character may be at once recognized by the position of 

 the gill-openings on the lower surface. 



The Rays have certain other features in common, and some 

 of these may be briefly mentioned. The body is always flattened 

 from above downwards — another characteristic feature of 

 creatures that live on the sea floor. The upper edges of the 

 eyes are never free ; that is to say, the skin of the eye-ball is 

 continuous with that of the head. There are never more 

 than 5 gill-clefts on each side. The dorsal fins are placed well 

 back, the first never being in front of the level of the pelvics, 

 and they are usually much reduced in size and may be alto- 

 gether wanting. There is never any anal fin. 



The spiracles, those vestiges of what were once functional 

 gill-clefts {see p. xix), are always large in the Rays, and are 

 generally placed close behind the eyes on the upper surface 

 of the head. In these fishes the spiracles have taken on an 

 entirely new duty, and one which has become necessary in 

 consequence of the change in the manner of life. When 

 breathing, a Shark takes in water through its mouth and 

 expels it through the gill-clefts. When swimming or crawling 

 about, a Ray is able to breathe in the same way, but when 

 resting on the bottom there is a danger of taking in sand or 

 mud with the water and thus of clogging the delicate gills. To 

 obviate this difficulty, when resting the Ray takes in water 

 through the spiracles, which are provided with special 

 movable valves rather like shutters, whose purpose is to 

 regulate the flow of water into the gills. 



