TPIE ELASMOBKANCH FISHES 



247 



the nervous collector (fig. 224) as in Heptanchus. This has been studied in a 

 number of forms by Braus (1898). Extremes of variation obtain in the num- 

 ber of nerves taking part in this collector. In primitive forms, as in Hep- 

 tanchus, the number may vary greatly and the collector may consist of multi- 

 tudes of strands wliich may or may not fuse together. A good example is 

 Chlamydoselachus (fig. 224), in which the twenty-fifth to the thirty-eighth 

 nerves take part. In other forms few nerves take part in its formation 

 (Spinax) or no collector as such is found (Squatixa, Eaja, fig. 223) . 



Fig. 225. Sympathetie nervous system, Scyllium canicula. (From Botazzi.) 

 d.a., dorsal aorta; s.f., sympathetic fibers to stomach (st.) ; s.g., sympathetic ganglia. 



The collector has been studied in great detail as to the relation wliich it 

 bears to the origin of paired fins. By those who hold to the gill-arch theory 

 of origin of the fins the collector is an argument for the posterior migration of 

 the pelvic fins, while those who accept the lateral fin-fold theory believe that 

 the collector shows principally that the paired fins formerly had a wider ex- 

 tent than they have at the present time. A greater extent is indicated further 

 by the fact that there may be a plexus posterior to the pelvic fin (fig. 224) . 



In Chlamydoselachus the posterior collector (fig. 224) comprises a num- 

 ber of segments. In the embryo of Acanthias a posterior collector is present, 

 although it is absent in the adult. The pelvic plexus in Eaja vomer is shown in 

 figure 223. 



SYMPATHETIC OR AUTONOMIC NERVOUS SYSTEM 



Although the sympathetic nervous system has been studied by many workers 

 but little is known concerning its form in the different Elasmobranchs. It ex- 

 tends from the region of the head to the posterior part of the kidney or meso- 

 nephros. 



The ciliary ganglion in the region of the oculomotor nerve represents the 

 sympathetic system in the head. In various Selachians one or more small gan- 

 glia are related to the third nerve. These in Acanthias are located near the 

 branching of the oculomotor nerve into its dorsal and ventral rami. The gan- 

 glion (or ganglia) gives rise to non-meduUated fibers which make up the short 



