| OCR Text |
Show 1876.] ANATOMY OF PLOTUS ANHINGA. 341 resembles that muscle in Sula and Pelecanus, being separated from the obturator externus by a well-marked interval, which is not the case in Phalacrocorax*. It is to be remembered, as I have had the opportunity of stating elsewheref, that in Fregata aquila the semitendinosus is entirely absent, as in the Accipitres, whilst in Phaethon it has an accessory head as well as a considerable bulk itself, these facts tending strongly to verify Brandt's division of the Steganopods % into three well-differentiated groups, of two of which the two above-named genera are the only examples. The alimentary canal of the Darter presents features of especial interest, as in its stomach there is a modification in the structure of the proventriculus not referred to in zoological works generally. It is fully described by M r . Macgillivray in Audubon's 'Ornithological Biography' §, where an excellent figure illustrates the account. Mr. Macgillivray also accurately describes most of the other viscera. He, however, omits to refer to the hairy mat in the second stomach, which latter viscus, he strangely says, is soft and smooth inside. The observations here made, which are in accordance with those of M r . Macgillivray, cannot be considered de trop, as the extremely abnormal conformation he describes required verification before it could be accepted as not being merely an individual peculiarity. The tongue, as an independent organ, does not exist. It is very small in all Steganopods, but free at its anterior extremity ; smallest proportionally in Pelecanus. In Plotus, however, it is not free at its apex, it forming merely a longitudinal groove along the middle of the floor of the mouth, and ending abruptly behind by a small transverse slightly projecting ridge, 2\ inches in front of the rima glot-tidis, which is evidently the rudiment of the base of the organ. The hyoid cornua, \\ inch long, running in the faucial membrane, here meet and blend. There is no crop; the oesophagus, however, is very dilatable. The proventriculus does not form a zone, as is the rule; nor does it form a patch, as in Struthio, Bhea, Chauna, and a few other birds; but it forms a special gland-cavity into which the individual constituents of the organ open. This cavity communicates with the digestive tube by a small orifice which is situated on the right side of the stomach, just below the commencement of the yellow dense characteristic epithelium of the stomach in birds. Plate XXVIII. fig. 2 will assist in rendering this explanation more distinct. The proventricular compartment is covered by peritoneum, is nearly globose, about the size of a chestnut, and fixed to the right side of the lower end of the oesophagus. On superficial inspection it looks very like an enlarged spleen (that organ being subgiobose in birds). Its cavity is very small, being much encroached upon by the great depth of the cylindrical glands which compose its walls. The yellow stomach-epithelium surrounds its orifice and goes no further. There are no indications of additional proventricular glands at the lower * Vide P. Z. S. 1873, p. 636. t P. Z. S. 1873, p. 636, and 1874, p. 122. X P. Z. S. 1874, p. 116. * § Vol. iv. p. 158. |
