GILLS 



247 



smaller compound vomerine teeth are set on the ethmoid cartilage 

 in front. They probably drop off in fossils, where they have never 

 actually been found. 



The pharynx leads into an oesophagus, stomach, and straight 

 intestine, fastened by an incomplete ventral as well as dorsal 

 mesentery. A well-developed spiral valve is present (Giinther [190], 

 Parker [324], Hyrtl [232]). The cloaca receives the anal, genital, 

 and urinary openings. Paired abdominal pores are present. 



Ceratodus has five open branchial slits. The first, behind the 

 hyoid arch, has a hyoidean hemibranch. It is a pseudobranch 

 supplied indirectly from the first epibranchial artery (Spencer [413], 





c.l.o 



FIG. 216. 



Larva of Ltpiiinxlrfn y/rf/<//i', Nat. (After Kerr, from Sedgwick's Zoology.) rl.o, cloacal 

 opening ; four external gills are shown, also the rudiments of the paired limbs, anrl the adhesive 

 organ. below the head. 



Kellicott [257]). The following four arches bear complete gills 

 (Figs. 207 and 220). 



Protopterus has also five open clefts, with a hyoidean hemi- 

 branch, two complete gills on the third and fourth arches only, 

 and a few gill-lamellae (an anterior hemibranch) on the fifth arch 

 (Fig. 221). Only four open gill-clefts remain in the adult Lepidosireii, 

 where the hyobranchial slit is closed. 



Larval gills are present in all recent Dipnoi ; in Protopterus and 

 Lepidosireii (Kerr [259], Budgett [67a], Semon [399]) they extend 

 freely to the exterior as branching processes from several gill- 

 arches (Fig. 216). In the former genus they may remain outside 

 the operculum in the adult (Fig. 221), and are supplied with 

 afferent and efferent vessels from the last three aortic arches. 



The complete gill-arches show in section (Fig. 57) a broad 

 septum beyond which the lamellae project a little. There are a 

 single afferent and two efferent vessels j but there is reason to 

 believe that the latter are not strictly comparable to those of 

 Elasmobranchs. Alone among the Osteichthyes, the Dipnoi have 



