1090 



SCAXDIXAVIAX FISHES. 



than in other tishes. Four kinds of organs belonging 

 to this system may also be distinguished: 1) the true 

 lateral line with its i-ainitications on the head, the 

 trunk, tlie pectoral tins, and the tail; 2) the so-called 

 Lorenzinian ampulke (niuciferous sac ducts); 3) the so- 

 called Savian vesicles; 4) the so-called pit organs. 



The lateral line, the histological structure of whicli 

 has been elucidated by Leydig (tig. 307), has nume- 

 rous ramifications, and opens on the surface of the body 

 not only into simple pores, but also into long trans- 

 verse branches originating at right angles or obliquely 

 from the line itself and its ramifications. The main 

 branches of the system indeed find their parallels in 

 the ramifications we have seen above in the Chim»i'a; 

 but peculiar to tlie true lateral line of the Rays is the 

 complex network of canals on the pectoral fins. In the 

 genus Raja we recognise on the ujiper surface of the 

 head the transverse occipital (supratemporal or aural) 

 branch (fig. 308, 21), the frontorostral branch with its 

 divisions, the occipital {20 to j) and the supraorl)ital 



Fig. 307. Part of a brand] of the cephalic system of the lateral line 



in the Thornhaclv (Raja clavata). Magnified. After Leydig. 



a, outer wall of hard membrane, removed on one side to show b, 



the inner, soft membrane, on which are visible the papilla (e) and 



the nerve centra (sensory spots, rf), with extended ramifications of 



the entering nerves (e). 



(/ to 2), and the suliorbital branch (7 to 8), which 

 runs on each side of the body between the eye and the 

 spiracle. When these branches have advanced nearly 

 to the margin of the body (at 2 and 6'), the former 

 quite close to the tip of the snout {2), the latter some- 

 what further back (-9), they pierce the bod}' to reappear 

 on the under surface of the head (2 to 6 and 8 to 11), 

 and join each other as in the Chimera. This junction 

 is simple, however, in the said species and follows the 

 median line of the snout (median canal, Garmax); here 

 it is double, one brancli on each side of the median 

 line, and the immediate continuation of the frontorostral 

 branch (supraorl)ital division) bends outwards on the 

 under surface of the snout in a sharp crook (from 3 

 to 4 and 5). After the junction the suborl)ital branch 



" LORENZINI, Observ. int. alh Torpcd., 1678, and Munro, Strii 



is contiinied backwards by a maxillary part (6 to 9), 

 that bends inwards, in about a line with the middle 

 of the length of the nasal valvule, to a naso-maxillo- 

 rostral branch (.9), which indeed (at 10) joins the cor- 

 responding branch of the other side to form a maxillo- 

 nasal canal, but also sends out a raaxillo-rostral canal, 

 .straight to the tip of the snout, within the continua- 

 tion of the supraorl)ital branch on the under surface 

 of the snout. The po-sterior and more immediate back- 

 ward continuation of the suborbital branch and its ma- 

 xillary part answers to the opercular canal of the Chi- 

 m:era, and has been named l)y Kwart the hyomandi- 

 bular branch (12 to 19). Tliis branch runs here (12 

 to 13) on the outside of the branchial apertures {aphr), 

 following the direction of tlie series formed by the 

 latter, and on the under surface of the pectoral fin 

 bends in a great loop, first backwards and outwards 

 {13 to 14), then almost straight forwards {14 to 15), 

 to a point near its origin, where it bends inwards {15 

 to Id) and afterwards runs forwards (1(1 to 17), pa- 

 i-allel to its commencement and to tlie suborbital branch, 

 until (at 17) it comes in a line with the nostrils, where 

 it turns upwards straight through the body. On the 

 dorsal side it now bends straight back (from 77 to 18), 

 and after an inward curve (at IS), where it receives 

 connecting ducts from the suborbital branch, it pursues 

 its course along the margin of the pectoral fin, and in 

 the posterior part of the dorsal side thereof (at 19) 

 joins a pleural Ijranch (22 to 23) from the lateral line 

 proper. The last-mentioned part of the system follows 

 on each side of the body the same course as in the 

 Teleosts, from the temporal region (at 21) to the tip 

 of the tail. At the shoulder- girdle it forms an outward 

 bend and here it gives off on the dorsal side of the 

 pectoral fin both the anterior pleural l>ranch -which we 

 have noticed above at its junction with the termination 

 of the hjomandibular brancii, and, in the genus Baja 

 and its nearest relatives, a posterior jjleural branch 

 (24 to 25), which ramifies in the posterior part of the 

 pectoral fin. Just behind the mouth there lies, as a 

 detached portion of the hyomandibular liranch, a trans- 

 verse canal (27), answering to the mandibular branch 

 of the Chimtera. 



The ampullary system (fig. 309) properly belongs 

 to the head alone, where all its canals have their ca?cal 

 base. These organs have been known for more than 

 three centuries", but their true nature was first eluci- 

 •(., Pli>/swl. Fish., 1785. 



