In the different stages of the nutritive process, certain substances are formed, which are sometimes essential to the animal economy, and at other times rejected as hurtful, these may be included under the general name of secretions. We shall successively advert to each of the subjects just enumerated.
Digestion.—As this function is almost entirely devolved on the organ named the alimentary canal, we shall endeavour, in the first place, to convey an accurate notion of the form and position of that important viscus. It may be described generally as an elongated tubular organ, occupying the centre of the body, and open at both extremities. Occasionally it is nearly straight and not longer than the body, but, in most instances, it is twisted on itself in numerous convolutions, and its length is consequently very considerable, sometimes twelve times as long as the body. In this respect it is found to vary, as among the higher animals, according to the nature of the food, being long and complicated in herbivorous species, and comparatively short in such as live by prey; but even this law is not without numerous and striking exceptions. In most cases the form is rendered irregular by many distentions and constrictions, which are so conspicuous that they may be regarded as dividing the canal into several parts, which have received different names according to the functions they perform. The place occupied by the canal is the median line of the body, immediately beneath the dorsal vessel. (See Pl. II. fig. 1, b, c, d, e, f.) Its texture is not the same throughout its whole extent, but its essential con-
