358 COMPARATIVE ANATOMY 



of the branchial cavities and skeleton. 1 These serve to retain 

 water and air, and thus the Fish is able to breathe for some 

 time out of the water (Anabas, Saccobranchus, Heterobranchus, 

 Clarias). 



The Dipnoi, as their name implies, possess both gills and 

 lungs, the latter alone being functional in Protopterus and 

 Lepidosiren during the torpid period (p. 20). The internal gills 

 are covered by a small operculum. In Ceratodns there are four 

 holobranchs on the first four branchial arches as well as a " hyoid 

 hemibranch " (which perhaps belongs to the first branchial arch) ; 



the gill-lamellae extend round the 

 clefts, so that the hemibranchs of 

 each cleft are continuous. In 

 Protopterus and Lepidosiren a re- 

 duction of these organs has taken 

 place, gills being absent in the 

 former genus, for example, on the 

 first and second branchial arches ; 

 there is, however, in addition, an 

 anterior hemibranch on the fifth 

 branchial arch. 2 



In embryos of Elasmobranchs 

 FIG. 264. EXTERNAL GILLS OF , -mi /rr 



LARVA OF Gymnarchw nilo- and certain leleosts (MeterotlS, 

 tifiis, 4 DAYS AFTER HATCH- Gymnarchus, Fig. 264), long, vas- 

 ING. (After J.S.Budgett.) cnlar , ; thread-like "external gills" 



DS, yolk sac ; KB, external gills, arise from the endoderm of the 



clefts and extend backwards over 



the body. In the larval Polypterus and Calamichthys there is a 

 single true (integumentary) external gill on either side in the 

 hyoid region, which differs markedly from those just described, 

 and consists of a main stem giving off a double row of filaments 

 and supported at its base by cartilage (Fig. 265, b). In larvse of 

 Protopterus and Lepidosiren somewhat similar external gills are 

 present, but are four in number 011 either side and are situated on 

 the branchial arches (Fig. 265, a). Of these, vestiges of the three 

 upper ones persist in Protopterus even in the adult. 



Amphibians. In the embryos of Urodeles, indications of five 

 gill-clefts can usually be recognised, but the most anterior (hyo- 

 mandibular) cleft, as in other Amphibians, does not become open 



1 Other parts may also become modified to serve as accessory respiratory 

 organs. Thus in Cobitis and Callichthys intestinal respiration takes place ; and 

 in Monopteriis javanensis (which, like Protopterus and Lepidosiren amongst the 

 Dipnoi, passes through a torpid period in holes in the ground during the dry 

 season) buccal, pharyngeal, and intestinal respiration occur, and interesting 

 modifications of the blood vessels are seen. 



2 Cf. Note on p. 97. There are five clefts in Ceratodus and Protopterus, 

 and four in Lepidosiren. It is probable that the hyobranchial cleft is closed in 

 Protopterus, as is the case in Lepidosiren; the spiracular cleft does not become 

 perforated in the embryo. 



