398 



Vertehrata. 



culunij and consisting of a stem and two rows of lamellse (Fig. 326). 

 These gills persist throughout life in the Perennibranchs, where 

 they are somewhat more complicated (branched) ; the embryos of 

 some Gymnophiona* possess similar gills {see Fig. 331). The 

 1 a r V se of the A n u r a also are furnished for a short time after 

 hatching with three pairs of external gills, like those of the larval 



rig. 326. Head, etc., of a, 

 XJrodelan larva, diagrammatic 

 (in the figure more of the thin 

 plates is seen than in reality, etc.). 

 h operculum, p thin plate on the 

 first giU bar. — Orig. 



Urodeles ; they are, however, soon covered by the operculum, which 

 is well developed, and covers gills and gill-clefts, concrescing 



posteriorly with the surface of 

 the body. A. large branchial 

 cavityt results, communicating 

 with the exterior by a single 

 aperture,^ usually on the left 

 side. The gills enclosed in this 

 cavity atrophy, and in their 

 place numerous branched "in- 

 ternal gills," structures 

 peculiar to the anuran larvae, 

 arise on the outer edge of all 

 the four bars . As in Selachians, 

 etc., there is usually an imperfect 

 straining apparatus at 

 the inner edge of the gill-bar, 

 in such forms as are provided 

 with external gills. It is repre- 

 sented on each arch by one or 

 two (one on the lirst and fourth 

 gill-bars, two on each of the 

 others) rows of short processes 



Fig. 327. A Young Tadpole of a 

 frog (lateral view) ; B somewhat older from 

 below; C stiU older larva with internal 

 giUs. 1, 2, 3 the three external gills, u. 

 anus, b hind limb, g branchial aperture, 

 inu caudal muscles, n narea, o mouth, op 

 operculum, s organ of adhesion. — C orig., A 

 and B with the assistance of figures by Eeker. 



* In other embryos of this division there is, Instead of such gills, one very vascular 

 lamina on each side. 



t The opercula of the two sides are continuous ventrally (Fig. 327 B), concrescing 

 posteriorly to enclose a single cavity. 



J In Pipa, and an allied genus, there are two openings, one on either side. 



