2G :srR. W. K. PARKER ON THE STRUCTURE AND 



outwards and forwards ; all the other clefts (c7-' ^~'^.} have the same form and direction, 

 but they are larger, and open externally, although covered afterwards vrith the oper- 

 cular skin, which only opens on the left side. The second cleft {cl~.) is between the 

 hyoid cornu and the first branchial pouch ; it runs into the floor some distance from the 

 hyoid facet. 



Between the articular surfaces of the free mandibles (ml:) the mucous membrane 

 is raised into two crescentic folds ; these are the lateral rudiments of the tongue (tg.) ; 

 if these lobes simply grew forwards the Frog would have a bilobate tongue with its 

 free ends growing forwards — it would be protero-glossal. 



Between these lobes there are several large papillae ; behind them the raucous 

 membrane thinly veils the hyo-branchial arches (Plate 1, fig. 4, and Plate 2, fig. 8). 



At the mid-line there are, behind the papillae, first the basi-hyal {b.hij.) and then 

 the double basi-branchial {h.hi:) ; whilst on each side are the broad hyoid cornua {c.hy.) 

 and the sub-median and lateral elements of the branchial arches {J/.I)r.) and its rays 

 — all that exists of the endoskeletal framework of these pai'ts. 'J'lms in the larval 

 condition the Frog has the dorsal portion of its hyoid and branchial arches suppressed : 

 four large " extra-branchials " (Plate 1, fig. 4, h-.jx ; and Plate 2, fig. 8, ex.hr^~\) 

 supplement the deficiency, and those bars, the first and last of which are cochleate, 

 are homologous with half of the branchial basket-work of the Lamprey, namely, witii 

 the four bars of its framework from the Jirst to the tJiird pouch inclusive. 



In his valuable paper on the Lamprey (Journ. of Anat. and Phys., vol. 10, pp. 412- 

 429), Professor Huxley does not notice the existence of arches homologous with the 

 proper branchials of Selachians, Ganoids, and Teleostei ; but his description of the cor- 

 resjjondence of these parts with those of the Lampi-ey must be given here : — 



" In the present stage the branchiae of the Tadpole are, as is well known, pouches, 

 which present no superficial likeness to the branchial sacs of the Lamprey. A septum 

 extends inwards from the concave face of each branchial arch, and the septa of the two 

 middle arches terminate in free edges in the branchial dilatation of the pharynx. 

 Vascular branchial tufts beset the whole convex outer edge of the branchial arch, and 

 are continued inwards in parallel transverse series of elevations, which become smaller 

 and smaller towards the free edge of each septum, near which they cease. 



"In the young Ammocoete the septa of the branchial chambers similarly bear vascular 

 processes, which are first developed close to the external branchial aperture, and thence 

 extend inwards transversely.* 



* In a note here, Professor IIuxi.ey remarks : " If the first-formed long brancliial filaments of the 

 Ammocajtc projected through the small gill-clefts outwards instead of inwards, they would resemble the 

 first-formed 'external gills' of Klasmobranchs. And this difference of direction seems to indicate tlie 

 solution of the difficulty, that external gills, which are so generally developed at first in Elasmohraiichii, 

 Oanoidci, and Dipnoi, are apparently wanting in Marsipohranchii." I may add to this note an observation 

 of my own illustrative of the writer's remarks, namely, that in the larva of Dacti/lethra there are no 

 branchial filaments growing from the outer face of the branchial pouches (see " Batrachia," Part II., 

 Pinte 5R, fig. 1 .) 



