[Illustration: FIG. 160.—The alimentary canal of a Perch (_Perca_). _an_, Anus; _in_, intestine; _oes_, oesophagus; _py_, pylorus; _py.c_, pyloric caeca; _st_, stomach. (After Wiedersheim.)] The precise function of these organs, whether digestive or absorptive, is still uncertain.[269] That they may be digestive is suggested by the presence of certain amylolytic and proteolytic enzymes, but this obvious conclusion is to some extent vitiated by the close proximity of these organs to the stomach, and more especially to the intestinal orifice of the pancreatic duct. It is by no means improbable, however, that the caeca are both digestive and absorptive organs. An attempt has been made to show that the pyloric caeca and the spiral valve vary inversely as regards the extent of their development in different groups of {276}Fishes.[270] To some extent the reciprocal variation of these structures supports this view, but it is also evident that there are obvious objections to its unqualified acceptance. Thus, in some Teleostomi (e.g. _Acipenser_, _Polyodon_), exceptionally well-developed and numerous caeca and a spiral valve are both present. _Amia_ with an almost vestigial spiral valve has no trace of pyloric caeca, and in Teleosts the absence of a spiral valve is associated with the complete suppression of the caeca in many large and important groups. THE RECTAL GLAND.—The "rectal" gland, or appendix digitiformis, is a small organ of unknown function with complex glandular walls, and a central duct opening dorsally into the terminal portion of the intestine.[271] The organ is generally present in Elasmobranchs (Fig. 153, _rct.gl_), in which group the intestinal orifice of its duct may either be close to the termination of the spiral valve, or, as in _Chlamydoselachus_,[272] near the cloacal outlet of the gut. An apparent representative of the gland, the "caecum cloacae," is also present in the Dipnoi,[273] but communicates directly with the cloaca (Fig. 155, A, _cl.c_). The "rectal" gland is perhaps homologous with the intestinal caecum which is to be found in some Teleosts (e.g. _Box vulgaris_), and possibly also with the "caecum" (caecum coli), and its vermiform appendix in the higher Vertebrata.[274] The caecum cloacae, on the contrary, is morphologically a urogenital sinus, formed as a dilatation of the fused hinder portions of the mesonephric ducts, and probably comparable with the sperm sacs of male Elasmobranchs, and also with the urinary bladder of Teleostomes.[275] {277}CHAPTER X THE RESPIRATORY ORGANS