140 



THE ELASMOBRANCH FISHES 



In development the valve first appears as a ridge or fold of the intestinal 

 mucosa along the intestinal wall. Whether the increase in the width of the 

 valve is due to the growth of the mucosa pulling the tissue within the folds as 

 in Ammocoetes (Daniel, 1931), or to the growth of connective tissue which 

 forces the mucosa downward as a cap, is not clear. The likelihood is that 

 both of these processes take place. This fold in a .young Zygaena hangs down 

 as a simple longitudinal plate which, upon reaching the opposite side of the 



lumen, rolls up into a scroll valve as would a sheet 

 of paper similarly droj^ped down. In the spiral 

 valve, however, another factor enters. Here a tor- 

 sion occurs in the lining of the intestine which 

 throws the scroll into a spiral. Figure 139 from 

 Riickert (1896) represents the mucosa of the in- 

 testine in the formation of the spiral. The turns in 

 figure 139b are more numerous in the posterior re- 

 gion, and the intestine in the older stage is seen to 

 be relatively shorter. 



The number of turns of the valve in the adult 

 varies greatly among the Elasmobranchs. In fact, 

 slight variation in the number within a single spe- 

 cies is common. In some the number is as low as 

 four (Prionace), or eight (Raja), or ten (Muste- 

 liis). In others it is increased but slightly (twelve 

 in Heterodontus). In some, however, great num- 

 bers are present, twenty in Lamna and forty-five 

 in Alopias. 



In width the valve in the adult may approach or 

 extend beyond the middle of the lumen [Hep- 

 ia7ichus). When it is less than one-half the width 

 of the intestine there appears in end view a canal 

 which would represent the core of the screw. If it 

 exceeds one-half the diameter of the intestine the 

 free edge representing the core of the screw rolls 

 up so as to appear from side view as a series of con- 

 centric cones pointing anteriorly or posteriorly 

 (Scyllium, fig. 138b). In some forms the valve anteriorly may thus be thrown 

 into cones, while posteriorly the free edge does not reach the middle line. 

 Hence an end view from the posterior would present a central lumen (Raja, 

 fig. 140). 



The lining of the valvular intestine from the duodenum to the end of the 

 spiral valve consists of cylindrical and goblet cells (Carcharias, Mustelus) 

 and is thrown into multitudes of tiny points or villi (fig. 140) . From the type 

 of lining it is apparent that the function of the spiral valve, in addition to pre- 

 venting the too rapid passage of food, is the absorption of digested substances. 



A B 



Fig. 139. Two stages A-B to 

 show the development of the 

 spiral valve, Prts^mrw^. (From 

 Riickert.) 



