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Show 1894.] YOUNG OF ECHIDNA ACULEATA. 5 My two specimens of this stage measure respectively along the dorsal curve from the end of the snout to the tip of the tail 21'5 cm. (see fig. 2) and 25-5 cm., the greatest diameter of the body being about 6 cm., and the head 4 cm. in length. The rough integument is covered with papillae, and the stiff bristles now project about 3 m m .; the position of the stronger and sparser spines amongst these can be seen more plainly than in the earlier stage, though they still hardly project above the surface. The snout is more plainly marked off from the rest of the head than in Stage I., and is relatively flatter: it is entirely covered with horn, and much resembles the " beak " of Ornithorhynchus except in the relative extent of the gape. The nostrils are now more completely valvular, and the caruncle is no longer recognizable. The eyelids are beginning to separate, the conjunctival chamber communicating with the exterior by a small aperture. The cloaca has become deeper, folds of the integument radiating out from the vent. The integument on the ventral side of the body is much folded ; and in the larger of the two specimens, which is probably a female, a shallow triangular pouch, the apex of which points backwards, can be seen between and rather anteriorly to the hind limbs. There can be little doubt that this represents the mammary pouch as described by Haackex • and as it is so distinct at this stage, it seems improbable that it would altogether disappear in the adult between the periods of suckling. I do not propose to treat of its structure or of its relation to the pouch of Marsupials in the present paper2. The pes is now nearly as large as the manus, though its claws are not so strongly developed. The calcaneal spur can be seen in both specimens, but is considerably larger in the smaller of the two, which is probably a male. II. Integument of the Head. The resemblance of the snout to that of Ornithorhynchus has already been remarked upon; and this is more particularly the case in the later stage, in which it is relatively flatter than in the younger one. As in Ornithorhynchus, the lips, as well as the whole integument of the snout, are immobile, owing to the development of a thick horny layer from the epidermis (Plates II. & III.). The horn is much thicker in the older of the two stages, and this is all the more remarkable as in the adult the skin in this region can hardly be said to be horny at all. The horny layer extends over the margins of the gape, and then thins off gradually : it also passes inwards to line the external narial passages (Plate II. fig. 4 and Plate III. fig. 13). The caruncle (figs. 1 and 4) is formed by a ridge of the epidermis on which the horn is especially thick3. 1 W . Haacke, " O n the Marsupial Ovum, the Mammary Pouch, and the Male Milk-glands of Echidna hystrix," Proc. Roy. Soc. vol. xxxviii. p. 72 ; and Biol. Oentralblatt, viii. . 2 Cf. H. Klaatsch, Morph. Jahrbuch, Bd. rvii. p. 483. 3 Cf. Carl Rose, " Ueb. die Zahnleiste und die Eischwiele der Sauropsiden," Anat. Anz. vii. Jahrgang, 1892, p. 748. |
