5i6 FISHES. 



were, parasitic upon itself, is probably of especial importance, not only 

 in the formation of leucocytes and in the destruction of dying cells, but 

 also in the processes of metabolism." 



Behind the hyoid are five rudimentary branchial arches. There are 

 five gill clefts, covered by an operculum, outside which are three 

 external epidermic gills. Of the true internal gills the arrangement 

 is as follows: — the hyoid has a small half row, the next two arches bear 

 none, the third and fourth have the usual double rows of lamellse, and 

 the fifth has a single row. 



The lungs are paired along almost their entire length, and extend under 

 the notochord to the end of the body cavity. The glottis lies as usual 

 on the median ventral floor of the pharynx, and by means of a vestibule 

 ascending on the right side communicates with the unpaired anterior 

 end of the lungs. Thus, although the lungs lie dorsally, they probably 

 arise as a ventral diverticulum, as in higher animals. 



The blood is remarl<able for the large size of its elements and for the 

 predominance of white over red corpuscles. In general structure the 

 heart is lil-;e that of Ceratodtis. There is but one auricle, but a dorsal 

 fibrous ridge hints at its division. The conus arteriosus has a long 

 spiral longitudinal valve and minute pocliet-like valves. From the cone 

 four branchial arteries arise on each side, and pass to the first four 

 branchial arches, and the effect of the longitudinal valve is that the 

 anterior pair contain blood already purified in the lungs ; the posterior 

 pair carry almost unmixed venous blood. The efferent branchials unite 

 in a transverse trunk, and then form the dorsal aorta, and from the root 

 of the aorta a paired pulmonary artery arises, the left supplying the 

 ventral, and the right the dorsal aspect of the lungs. In regard to the 

 veins, there is a single true postcaval, or inferior vena cava, along with 

 a persistent left posterior cardinal. There is a single caudal vein gi%"ing 

 rise to a right and left renal portal. Two pulmonary veins unite near 

 the front of the lung in a single vessel, which enters the left side of the 

 auricle. 



The urinogenital organs are surrounded by lymphoid and fatty tissue ; 

 the kidneys probably represent the mesonephros, and their duct the 

 Wolffian duct ; nephrostomes are absent. The vas deferens appears to 

 be a special duct, probably formed in connection with the testes, quite 

 independently of the excretory apparatus, and, therefore, to a certain 

 extent comparable to that of Teleosteans ; it opens into the base of the 

 IMlillerian duct, the rest of which gradually aborts in the male. The 

 ovaries are strikingly like those of Amphibians ; the o\-iduct seems to 

 be the Miillerian duct. Ureters and genital ducts open beside one 

 another into the cloaca. 



{c) Lepidosiren. — Relatively little is kno«n in regard to the third 

 type — Lepidosiren — from the Amazons. It has an eebshaped body with 

 a continuous vertical fin. The limbs are reduced to cylindrical stems 

 without any radials. There are no external gills. The air bladder or 

 lung is double, and its relations to blood vessels are like those in Proto- 

 pterus. 



There is an imperfect muscular septum dividing the auricle into two, 

 and there is a similarly incomplete septum in the ventricle. The conus 

 resembles that of Protoptems. 



