723. In like manner, in some very simple arrangements of cæca and tubuli, each body opens by its own distinct orifice (fig. CLXXXV. 2). But in the more complex arrangements of these bodies, it is indispensably necessary to modify this mode of parting with their contents. When the elementary bodies are aggregated into dense, thick masses (fig. CLXXXIX.), when layer after layer of these masses containing myriads of myriads of follicles, cæca, or tubuli, are superimposed one upon another, (fig. CLXXXIX.), it is impossible that each individual body can have a separate orifice. In this case a minute tube springs from each body (fig. CLXXXVI. 2); and a complete connexion is established between all the individuals composing the mass by the free intercommunication of these tubes (fig. CLXXXVI. 2). Of these tubes the minutest unite together, and form larger branches (fig. CLXXXVI. 2); these larger branches again uniting form still larger branches (fig. CLXXXVI. 2), until, by their successive union, the branches form at length a single trunk (fig. CLXXXVI. 3), with which all the individual branches, whether great or small, communicate, and into which they all pour their contents (fig. CLXXXII. 2, 3). The bodies from which these tubes take their origin, and the minute tubes themselves, are called secreting canals (fig. CLXXXII. 1, 2); the common trunk formed by their union is termed the excretory duct (fig. CLXXXII. 3). The secreting canals contain the secreted matter; the excretory duct collects this matter, and conveys it to the part of the body in which it is appropriated to the specific purpose which it serves in the economy. 724. The basis of the secreting canals consists, then, of membrane disposed in one or other of the elementary forms described (712, _et seq._), These secreting canals constitute a peculiar system of organs wholly different from all the other organs of the body. The form of these organs, their structure and their relation to the blood-vessels and nerves, have formed subjects of laborious investigation and of keen controversy during several centuries. The honour of discovering the exact truth on these points is due to very recent researches.