312 THL VITAL FUNCTIONS. 



are often scattered throughout the surfaces of mem- 

 branes:* 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 vrhich we have already 

 seen in the hepatic and salivary glands of insects. 



A secretory organ, in its simplest form, consists 

 of short, narrow and undivided tubes ; we next find 

 tubes which are elongated, tortuous or convoluted, 

 occasionally presenting dilated portions, 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. t The secreting^ fila- 

 ments of insects float in the general cavity, con- 

 taining the mass of nutrient fluid, and thence im- 

 bibe the materials they require for the performance 

 of their functions. It is only when they receive 

 a firm investment of cellular membrane, formins; 

 what is termed a capsule, and assuming the appear- 

 ance of a compact body, that they properly consti- 

 tute a gland; and this form of a secreting organ is 

 met with only among the higher animals.| 



Great variety is observable both in the form and 



* Seep. 165 of this volume ; and in particular Fig. 305. Sfeba- 

 ceous follicles are also noticed in vol. i. p. 102. 



t See " Muller on the intimate structure of secreting glands," 

 edited by S. Solly, 1839. 



X Dr. Kidd, however, describes bodies apparently of a glandular 

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

 canal of the Gryllolalpa, or mole-cricket. Phil. Tran. for 1825, 

 p. 227. 



