NOTES ON MEDIAN AND PAIRED FINS OF FISH. 363 



is unchanged. Much more pronounced is the apparent shifting 

 backwards of the dorsal fins. While their hinder margin is only 

 slightly carried forwards, the anterior margin retreats over 

 some eight or nine segments. Here, again, there are fixed 

 points about the fortieth and fifty-fourth nerves which do 

 not move at all (j)p. 343-352). 



On the Nerve-Supply op the Fins. 



We must now more closely examine the structure of the 

 plexus of nerves which supply the fins, dealing more particu- 

 larly with the median fins. 



Mayer (22), to whom I am much indebted for many useful 

 hints on the best methods for this purpose, has described and 

 figured the nerve-plexus of the median fins of many Elasmo- 

 brancli fish. But he did not follow out in detail the relation 

 of the nerves to the fin muscles and to the body segments. 

 With the object of continuing and extending his researches, I 

 have dissected the nerve-plexus in a large number of speci- 

 mens. For this purpose material has been used after treat- 

 ment with hot water, or after maceration in weak nitric acid. 

 Osniic acid added to these preparations brings out the nerves 

 most distinctly. Owing to the delicate and complex nature 

 of the plexus of the median fins and to the very brittle state 

 of the nerves, it is very difficult indeed to obtain a perfectly 

 complete dissection of the plexus in a single specimen. The 



EXPLANATION OP TEXT-FIGURES. 



Text-figure 1. — Diagram of an adult Scy Ilium canicula, showiii"' t.lie 

 nerve-supplj of the fins. 



Text-figure 2.— Diagram of an adult Scyllium canicula. The fins are 

 expanded, and their nervous, muscular, and skeletal segmental elements are 

 distributed as if concentration had not taken place. The nerve foramina in 

 the girdles are indicated by shaded oval areas ; the girdles themselves are not 

 shown. 



Text-figure 3. — Diagram of an embryo Scyllium canicula about 19 mm. 

 long, in which are shown the ganglia, the myotomes, and the muscle-buds. 

 <r, Anal fin ; ac, anterior collector of first dorsal fin ; cr, cartilaginous radial 

 projecting beyond the radial muscles; n 1-57, spinal nerves and gano-lia; 

 pc^ collector nerve of second dorsal fin; pi, pelvic fin; pt, pectoral fin; rm, 

 radial muscle; \d and Id, first and second dors^al fins. 



