719. 5. And, lastly, the cæcum itself is elongated; so that instead of presenting the appearance of a pouch, it rather resembles a tube (fig. CLXXXV. 1), and is accordingly named tubulum. 720. In the construction of the secreting apparatus, membrane, then, may be said to be disposed into four elementary forms constituting cryptæ or vesicles, follicles, cæca and tubuli. Membrane, disposed into these elementary forms, constitutes the simple bodies by the accumulation and the varied arrangement of which the compound organs are composed. There is no other known element which enters into the composition of the most complex secreting organ. 721. One of these elementary bodies may exist as a simple organ, or many may be collected into a mass to form a compound organ. When single they are called solitary: when collected into a mass, aggregated. Each elementary body has a mode of aggregation peculiar to itself. Vesicles aggregate by clustering together (fig. CXXXVIII.), and adhering as if by a common stem (fig. CXXXVIII.); follicles by uniting at their orifices (fig. CLXXXIII.), and forming masses which are disposed either in a linear direction (fig. CLXXXIII.) or in fasciculi (fig. CLXXXIV.); cæca by forming bundles, parallel or branched (fig. CLXXXVI.); and tubuli by forming masses straight (fig. CLXXXV.), tortuous or convoluted (figs. CLXXXV. and CLXXXIX.). [Illustration: Fig. CLXXXIII. Aggregated follicles disposed in a linear direction, here represented of their natural size, as seen near the mouth in the goose.] [Illustration: Fig. CLXXXIV. Conglomerated follicles.] 722. When a single elementary body, as a vesicle or follicle, forms a distinct secreting organ, the matter secreted is elaborated at the inner surface of the organ (fig. CLXXXII. 2), and is contained within its cavity. When needed it quits this cavity through the walls of the vesicle, or at the orifice of the follicle, on the application of the appropriate stimulus. When a number of cryptæ or vesicles are aggregated into clusters, the individual vesicles sometimes open by distinct orifices into a common receptacle or sac (fig. CLXXXIV.). When follicles are aggregated into a mass, and the mass is disposed in a linear direction (fig. CLXXXIII.), each follicle pours out its secreted matter by its own orifice (fig. CLXXXIII.); but if conglomerated, into a common mass by a common orifice (fig. CLXXXIV.). [Illustration: Fig. CLXXXV. 1. Parallel tubuli, opening by distinct orifices into—2. A common cavity.] [Illustration: Fig. CLXXXVI. Branched cæca, showing—1. The cæca terminating in—2. Excretory ducts which unite to form—3. A common trunk.]