526 PISCES— FISHES. 



with posterior as well as anterior nares — the latter concealed by the 

 overhanging lips — relatively small, lidless eyes, and auditory organs. 

 " The apparently anomalous position of the nostrils is probably to be 

 explained as an adaptation to the habits of the animal in connection 

 with its summer sleep. " 



There is a spiral valve in the large intestine ; the cloaca has an 

 associated "caecum"; the pancreas surrounds the bile-duct, and, though 

 large, is almost hidden within the walls of the gut ; the spleen is also 

 large, but inconspicuous. Cilia are present throughout the stomach and 

 intestine, and there are no differentiated gastric or intestinal glands. 

 There is an unusually abundant investment of lymphoid tissue associated 

 with the gut, "which, during the period when Protopterus is, as it 

 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 process 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 remarkable for the large size of its elements, and for the 

 predominance of white over red corpuscles. In general structure the 

 heart is like that of Ceratodus. 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 pocket-like valves. From the 

 conus 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 cany 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 giving 

 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 urogenital 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 

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



