In an older embryo the fish-like form may be recognized. The medullary folds have completely fused in the median line, and the embryo is coming to acquire a ridge-like prominence; optic vesicles and primitive segments are apparent, and the blastopore appears to persist as the anus. The continued growth of the embryo above the yolk mass is apparent; the head end has, however, grown the more rapidly, showing gill-slits, auditory, optic, and nasal vesicles, at a time when the tail mass has hardly emerged from the surface. Pronephros has here appeared. It is not until the stage of the late embryo that the hinder trunk region and tail come to be prominent. The embryo's axis elongates and becomes straighter; the yolk mass is now much reduced, acquiring a more and more oblong form, lying in front of the tail in the region of the posterior gut. The head and even the region of the pronephros are clearly separate from the yolk-sac; the mouth is coming to be formed.
According to Eastman (Ed. Zittel), the skeleton of Neoceratodus is less developed and less ossified than that of its supposed Triassic ancestors. A similar rule holds with regard to the sturgeons and some Amphibians.
Fig. 388.—Adult male of Lepidosiren paradoxa Fitzinger. (After Kerr.)
Lepidosirenidæ.—The family Lepidosirenidæ, representing the suborder Diplopneumona, is represented by two genera of mudfishes found in streams of Africa and South America. Lepidosiren paradoxa was discovered by Natterer in 1837 in tributaries of the Amazon. It was long of great rarity in collections, but quite recently large numbers have been obtained, and Dr. J. Graham Kerr of the University of Cambridge has given a very useful account of its structure and development. From his memoir we condense the following record of its habits as seen in the swamps in a region known as Gran Chaco, which lies under the Tropic of Capricorn. These swamps in the rainy season have a depth of from two to four feet, becoming entirely dry in the southern winter (June, July).
Fig. 389.—Embryo (3 days before hatching) and larva (13 days after hatching) of Lepidosiren paradoxa Fitzinger. (After Kerr.)
Kerr on the Habits of Lepidosiren.—The loalach, as the Lepidosiren is locally called, is normally sluggish, wriggling slowly about at the bottom of the swamp, using its hind limbs in irregular alternation as it clambers through the dense vegetation. More rapid movement is brought about by lateral strokes of the large and powerful posterior end of the body. It burrows with great facility, gliding through the mud, for which form of movement the shape of the head, with the upper lip overlapping the lower and the external nostril placed within the lower lip, is admirably adapted. It feeds on plants, algæ, and leaves of flower-plants. The gills are small and quite unable to supply its respiratory needs, and the animal must rise to the surface at intervals, like a frog. It breathes with its lungs as continuously and rhythmically as a mammal, the air being inhaled through the mouth. The animal makes no vocal sound, the older observation that it utters a cry like that of a cat being doubtless erroneous. Its strongest sense is that of smell. In darkness it grows paler in color, the black chromatophores shrinking in absence of light and enlarging in the sunshine. In injured animals this reaction becomes much less, as they remain pale even in daylight.
Fig. 390.—Larva of Lepidosiren paradoxa 30 days after hatching. (After Kerr.)