156 



THE ELASMOBRANCH FISHES 



DIEECTION OF RESPIRATOEY CUREENT 



Fig. 149. Sagittal section through buccal cav- 

 ity of Baia erinacea to show valves (v.). (From 

 Eand.) 



In the free-swimming sharks the current enters the mouth, from which it 

 passes through the pharynx and into the gill pockets, the external clefts, in- 

 cluding the spiracle, at the same time remaining closed. The mouth then closes, 

 the external clefts open, and the water is forced out. 



In the rays, which spend most of their time on the bottom and hence often 

 in mud or sand, there is an interesting change in the direction of the current. 

 In these the greater part of the current enters through the spiracle and but 

 little through the mouth. The valve of the spiracle then closes and the water 

 is forced out ventrally through the external branchial clefts. At the expulsion 

 of the water the mouth does not entirely close, but only a little of the current 



is able to gain exit through it because 

 of valves wdiich are located on its 

 roof and floor (v., tig. 149). 



In ra^ys there often occurs a rever- 

 sal of the current, by which the water 

 is spouted outward through the spir- 

 acle. Rand (1907) has shown that 

 this may be brought about experi- 

 mentally in several ways. In the first 

 place spouting may be produced by 

 putting the ray into water over- 

 charged with carbon dioxide, or it may be the result of fatigue as is shown by 

 compelling the ray to keep in rapid motion for a period of time. Again, spout- 

 ing may be produced by putting a soft substance, as for example sea moss, into 

 the spiracle. To any of these experiments the ray responds by ejecting a col- 

 umn of water through both spiracles. Rand has shown further that by striking 

 the margin of the spiracle or of the eye on a single side, spouting may be pro- 

 duced by a single spiracle on the side thus irritated. 



While spouting is characteristic of the rays it is not confined to them. In 

 Squatina, a form which spends much time on the bottom, I have also found 

 spouting to occur. But in this type it is not so much an indication of a reversal 

 of the current. Here as in other sharks water enters the mouth and passes 

 out through the clefts. To study the spouting behavior and to note the direc- 

 tion of the current, I have observed Squaiiyia (a shark) and Rhinobatis (a ray) 

 in the same large aquarium. Under such conditions both are seen to spout 

 occasionally. With exercise the spouting occurs more frequently. If now the 

 water in the aquarium is let off so as to expose the spiracular clefts, Rhino- 

 batis becomes greatly agitated while Squatina is little disturbed. These 

 observations show that the spiracle in Squatina is not, as it is in the rays, the 

 principal intake. Darbishire (1907) says for Squatina, however, that carmine 

 liberated in the region of the spiracle enters, and furthermore, that it enters 

 not rhythmically but in a constant stream. This regularity of the current is 

 produced by a rhythmic action of the free margins of the gill septa. In the 



