APODES. 223 



Sub-order 4. APODES (ANGUILLIFORMES). The Eels. 



The premaxillaries small or absent, the maxillaries lateral, 

 the body eel-like and without pelvics. Symplectic absent ; 

 operculum and palatine arch reduced ; scales absent or 

 feeble ; pectoral arch not attached to skull ; fins with- 

 out spines, median fins if present confluent ; no pseudobranch ; 

 tail protocercal ; no pyloric caeca ; no generative ducts. Air- 

 bladder, when present, with a ductus pneumaticus. 



The eels are spread over the f. ws. and seas of the 

 trop. and temp, zones ; some descend to the greatest 

 depths. The young of some have a limited existence and are 

 known as Leptocephalus (see below). Fossil Anguilla in chalk 

 of Aix and Oeningen, Anguilla, Sphagebranchus, Ophichihys 

 at Monte Bolca and Urenchelys S. Wood., with homocercal 

 tail, from the chalk. 



The breeding * of the common eel was until a short time ago a mystery. 

 During their sojourn in freshwater they do not develop reproductive 

 organs, and it was not known how they originated. Aristotle thought 

 that they came from the " entrails of the earth." It is now known, thanks 

 to the researches of Grassi and Calandruccio, that they breed in the depths 

 of the sea, that the eggs float but remain near the bottom, and that they 

 hatch out as a larva, which soon becomes transformed into a ribbon- 

 shaped, transparent creature, which has long been known and called 

 Leptocephalus. There are several kinds of Leptocephalus. That of the 

 common eel is L. brevirostris. It appears to remain at the bottom, pro- 

 bably hiding under stones or burrowing in sand and mud until it meta- 

 morphoses into the elver. Elvers (see below) are the young of eels which 

 ascend rivers in great numbers. 



The Italian naturalists worked at Catania in the Straits of Messina, 

 where specimens of the Leptocephalus brevirostris are common in certain 

 years at the surface, and at all times in the stomach of Orthagoriscus mola, 

 a deep-sea fish, and they showed that this particular kind is the larva of the 

 common eel. That it should be taken here and nowhere else is a curious 

 fact, considering that the common eel is widely distributed. The probable 

 explanation is that it is brought to the surface by the currents and whirl- 

 pools which abound in this locality, while elsewhere it has escaped observa- 

 tion by lurking at considerable depths (300 fms.) in mud and under stones. 

 Several species of Leptocephalus, which doubtless belong to different 

 Muraenidae, are known as pelagic forms, especially in the tropics, so that 

 it is probable that all Leptocephali are not confined to deep water during 

 their development. Speaking generally it appears that female Murae- 

 noids cannot mature their ova except in deep water, while the male can 



* B. Grassi and S. Calandruccio, Ulteriori recerche sulle metamorfosi 

 dei Murenoidi, Rend. Ace. Lincei (5), vi., p. 43, 1897 ; also Q. J. M. S.,39, 

 1897, p. 371, and Proc. Roy. Soc., 1896. Cunningham, Journal of Marine 

 Biological Assoc. (2), 3, 1895, p. 278, and (2), 1, 1891, p. 16. 



