348 THE VITAL FUNCTIONS. 



throughout the surfaces of membranes :* at other 

 times the secreting cavities are collected in great 

 numbers into groups ; and they then frequently 

 consist of a series of lengthened tubes, like caeca, 

 examples of which we have already seen in the 

 hepatic and salivary glands of insects. 



A secretory organ, in its simplest form, con- 

 sists of short, narrow and undivided tubes ; we 

 next find tubes which are elongated, tortuous or 

 convoluted, occasionally presenting dilated por- 

 tions, or even having altogether the appearance 

 of a collection of pouches, or sacs ; while in other 

 cases they are branched, and extend into minute 

 ramifications. Sometimes they are detached, or 

 isolated ; at other times they are collected into 

 tufts, or variously grouped into masses, where still 

 the separate tubes admit of being unravelled. The 

 secreting filaments of insects float in the general 

 cavity, containing the mass of nutrient fluid, and 

 thence imbibe the materials they require for the 

 performance of their functions. It is only when 

 they receive a firm investment of cellular mem- 

 brane, forming what is termed a capsule, and 

 assuming the appearance of a compact body, 

 that they properly constitute a gland; and this 

 form of a secreting organ is met with only among 

 the higher animals. t 



* See p. 185 of this volume; and in particular Fig. 305. 

 Sebaceous follicles are also noticed in vol. i. p. 114. 



t Dr. Kidd, however, describes bodies apparently of a glan- 

 dular character, disposed in rows on the inner surface of the 



