. Outlines of zoology. Zoology. 552 AMPHIBIA. external gills soon become branched. There are three of them on each side, the first the largest. The mouth, which has previously been merely a blind pit, opens into the gut, the gut itself lengthens rapidly, and be- -na comes coiled like a watch-spring; 1"° the larvae feed eagerly on veget- ^JD able matter and increase in size. The glandular crescent forms two small discs, which gradually dis- appear as the power of locomotion increases. About the time when the mouth is opened, four gill- clefts open from the pharynx to the exterior. A second
. Outlines of zoology. Zoology. 552 AMPHIBIA. external gills soon become branched. There are three of them on each side, the first the largest. The mouth, which has previously been merely a blind pit, opens into the gut, the gut itself lengthens rapidly, and be- -na comes coiled like a watch-spring; 1"° the larvae feed eagerly on veget- ^JD able matter and increase in size. The glandular crescent forms two small discs, which gradually dis- appear as the power of locomotion increases. About the time when the mouth is opened, four gill- clefts open from the pharynx to the exterior. A second period, the true tad- pole stage, now begins. A skin-fold or operculum covers the external gills, which then atrophy, and are replaced by internal gills developed on four branchial arches. The mouth acquires horny jaws, and the fleshy lips bear horny papillae. By the continued growth of the opercular folds the gill-chambers are closed, with the exception of a single exhalant aperture on the left side. Through this opening, the water which is taken in by the „ , mouth in respiration passes out- Fig. 238.—Dissection of tad- j i_ • , ,;. .„ pole?- After Milnes Mar- wards, having washed the gills on shall and Bles. its way. dl., lower lip; #., ventricle of in tne third period the rudi- heart; Z> oesophagus; NA., , f head kidney; a., aorta; k., ments or the limbs appear. The ™»ZU%%T!&-, kv.; fore-limbs are concealed within the opening of ureter into cloaca; gill-chambers, and so are not GR., genital ridge; , fatty 1 • .., , , . body; lf., forf-iimb; oa, in- obvious until a later stage; but SLg'"s; "' epidermis; *' the hind-legs may be watched in the progress of development from small papillae to the complete Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Thomson, J.
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
