32 Scientific Proceedings, Royal Dublin Society. 



Dohrn suggests that the ciliated grooves in Ammocoetes are mucus- 

 excreting glands. The mucus, in enveloping food, is supposed to 

 protect the delicate walls of the intestine, and also to act on it 

 chemically. 



The endostyle of Amphioxus is not a groove, but a raised ridge. 

 Histologically, however, this difference between tunicates and 

 Amphioxus disappears entirely, and the homology of the organ with 

 the thyroid of Ammocoetes and the hypobranchial groove of tuni- 

 cates becomes evident. 



IX. — The Signification of the Unpaired Fin, fyc. x 



In a previous chapter Dohrn explained that he had not succeeded 

 in demonstrating the derivation of the muscular system in the 

 unpaired caudal fin of elasmobranchs. In teleosteans, however, 

 this is easily shown. On a row of myotomes or mesoblastic somites, 

 lying immediately behind the anus, muscle -buds grow out, which 

 form the muscular-system of the so-called anal fin. If we can 

 demonstrate, says Dohrn, that the unpaired fin has been produced 

 by coalescence of paired rudiments, and that these paired rudiments 

 are found in cyclostomes, we likewise prove that these must have 

 possessed pectoral and pelvic fins like other fishes. Indeed, if 

 Ammocoetes is examined with regard to the origin of its muscles, 

 muscle-buds are found both dorsally and ventrally, but they re- 

 main as indifferent cells during the Ammocoetes stage. During 

 the later stages the buds are differentiated into the fin-muscles. 

 These buds in Petromyzon are therefore homologous with the 

 dorsal and ventral muscle-buds, which in elasmobranchs and 

 teleosteans produce the paired and unpaired fins. As they 

 develop, especially on the posterior part of the back, into muscles of 

 the unpaired dorsal fin, it follows that the present ventral ones must 

 also at one time have formed muscles of paired fins. No doubt 

 they have become lost during the general process of degeneration 

 and reduction which the cyclostomes have undergone. Dohrn 

 found no trace of the pectoral fin, except the muscle-buds ; however, 

 the longitudinal folds bordering the anus in Petromyzon may be 



1 Ibid., vol.vi. 1885, pp. 399-431. 



