280 



THE ELASMOBRANCH FISHES 



groups {isa. and mpo.). In addition to these there is a modified ampulla in 

 the spiraeular wall. In active forms the pores to these organs may be very 

 numerous as in Mvstelus cams in which practically 1600 have been counted. 

 In a sluggish type like Torpedo there are as few as 162 (Norris, 1929). 



Each ampullar}^ organ (fig. 246) consists of three parts: (1) a pore or 

 opening to the exterior (op.) ; (2) a canal or tubule {th.) ; and (3) the ampulla 

 proper (a.), located in the in- . - 



tegument. The ampulla varies 

 as to pattern in the different 

 Elasmobranchs. In some it is 

 not divided up into ampullary 

 pockets. In others it may have 

 from eight to twelve pockets. 

 These pockets are usually con- 

 nected by a single canal with 

 the outside pore. In Hexan- 

 clius, however, Dotterweich 

 (1932) has recently shown that 

 each ampullary pocket in a 

 group has its ovn\ canal and 

 that a group of canals empties 

 by a common pore. A trans- 

 verse section through the am- 

 pulla (fig. 247a) shows how 

 they and the partitions sepa- 

 rating them are cut. Accord- 

 ing to Peabody (1897) each 

 ampullary pocket ipl'.) has 

 a double lining, the inner layer 

 of which is of cells of large size. 

 The pockets are surrounded by 

 connective tissue and maj^ 

 themselves surround a central part, the ampullary centrum {en.). Figure 247b 

 is a sagittal section through the centrum. The section is through a pocket on 

 the left and a partition between two pockets on the right. In figure 248 it is 

 seen that the nerve to the ampulla enters through the centrum and spreads out 

 over the ampullary pockets. 



It was formerly supposed that the cap over the centrum was the sensory 

 area in which the nerve terminated, but in figure 248 it appears that, while 

 the nerves lose their medullary sheaths and only the axis cylinders run toward 

 the central cap, the fibers turn outward as the fibrils surround the ampullary 

 sacs. Dotterweich (1932) has recently shown that the wall of an ampulla of 

 Lorenzini is made of a single layer but that this layer consists of two impor- 

 tant types of cells. One of these is a flask-like cell {g., fig. 249) which forms 

 mucus. The other is a pyramidal or sensory cell (s.). These pyramidal re- 



Fig. 246. Ajiipullary 

 organ of Lorenzini. 

 (From Peabody.) 



a., ampulla; ap., 

 aperture to outside ; 

 tl)., tubule. 



Fig. 247. A. Transverse section. 

 B. Sagittal section of an am- 

 pulla, Ga/eifs. (From Peabody.) 

 en., centrum ; pi:., ampullary 

 pocket. 



