132 UREEDING-IIABITS OF SOME WEST-AFHICAN FISHES. 



nest of Gijmnarchus, notwithstanding that the cgi>;s and larv;c wore lying within six 

 inches of tlie snrface of the water, quite unprotected from the burning rays of a 

 tropical sun and the lasliing of the tropical rains. The extremes of temperature 

 taken in tiie nests were 25° C. and 32'-5 C. But supposing tlie larvte to be so con- 

 stituted tliat they can withstand the changes of the weather, how is it that large 

 conspicuous eggs in very conspicuous nests on the surface of tlie water escape forming 

 the food of the abundant bird-life of these swamps ? 



In the breeding-habits of the last four types I have described, the interesting 

 fact comes out that the first two and the last two each have, in common, organs in 

 the larva which are usually regarded as not belonging to the Teleosteau division of 

 fishes. Gymnarchus and Heterotis have each, for a time, enormously elongated gill- 

 filaments, structures which are so characteristic of Elasmobranch larvae. Something 

 of the kind was noticed in the development of the loach by Gotte ^ but I think 

 this is the only case of such organs in the Teleosteans. Sarcodaces and Ilijperopisus 

 have each well-developed cement-organs on the head. These structures are generally 

 regarded as characteristic of the Ganoids. It seems, then, that the conditions by which 

 fishes, which breed in tropical fresh waters, are surrounded is conducive to the 

 development of very various accessory organs in the larva, both for the purpose of 

 respiration and also of preserving them from harmful contact with their surroundings, 

 and that these structures cannot be regarded as having any great morphological 

 meaning. The resemblance of the embryo of Gxjmnarchus to that of an Elasmobranch 

 I hope to discuss in a future work on the development of Gymnarchus. 



IX.— EXPLANATION OF THE PLx\TES. 



All the figures of Plate X. were originally drawn by myself with the aid of a camera lucida, and 

 were tlien copied by Mr. Edwin AVilson. 



Figs. 2, 3, 4, 5, 12, & 13 were drawn on the Gambia from living specimens. Figs. I, 6, 7, S, 

 9, 10, & 11 were drawn from formalin specimens. 



The magnification is 8 diameters. 



The figures in Plate XI. were all drawn by myself except fig. 1 , which was from a specimen 

 preserved in formalin, drawn by Mr. Edwin Wilson under my supervision and coloured from my 

 notes by myself. Figs. 4, 5, 6, 7, & 8 were drawn from life on the Gambia. Fig. 3 was drawn 

 from a specimen preserved with corrosive sublimate and acetic acid. 



au., auditory sac ; bp., blastopore ; hr., branchial eminences ; br. i. ii. &c., branchial arches ; 

 C.U., cement-organ ; cl., position of cloaca ; ep.e., growing edge of epiblast ; E(/. i. ii. ik,c., external 

 gills;//., dorsal fin-fold; A.Z., hind limb ; //"., hyoid arch ; /:/.6r., hyobranchial cleft; invag.,\uie 

 of invagination; M., mandibular arch; M.H., mandibulo-hyoid fold; m.f., medullary folds; 

 O.C., optic outgrowth from brain ; op., operculum; p.f., pecloral fin ; /j?i., pronephros ; s.v.g., groove 

 marking rudiment of spiral valve; sp., groove between mandibular and hyoid arches; imv., fourth 

 ventricle ; y.k., yolk-cells. 



' " Entwick. d. Teleostierkeime," Zool. Anz. Xo. 3, 1S7S. 



