220 LECTURE XVII. 



fine membraneous, or peritoneal layer, a compact muscular mem- 

 brane, and an internal mucous or epithelial coat. Between the last 

 two there is a white spongy layer of cellular tissue, compared by 

 Ramdohr to transuded chyle, and which is sometimes the seat of 

 gastric glands. 



The several divisions and convolutions of the alimentary canal are 

 supported and attached to the adjoining parts by the air-vessels : 

 there is no mesentery. 



At least three kinds of glands add their secretions to those of the 

 caeca and follicles of the alimentary canal. The first kind open in or 

 near the commencement of the canal, and are regarded as salivary 

 glands. They are classified by Professor Burmeister as follows : — 



A. Salivary vessels which open into the mouth, generally beneath 

 the tongue, sometimes at the base of the mandibles. They take the 

 following forms : — 



1. As simple, long, undivided, twisted tubes ; thus in the majority 

 of insects, viz. all butterflies, many beetles, and flies. 



2. As a narrow vessel which empties itself into one or two 

 bladders, whence the salivary duct originates {Nepa, Cimex^ Sarco- 

 phaga). 



3. As a ramose vessel with blind branches (Blaps). 



4. As two long cylindrical pipes, which unite into one excretory 

 duct (Heduvius). 



5. As four small, round bladders, each pair of which has a common 

 duct (^Pulex, LygcBus^ Cimex). 



6. As a multitude of such vesicles (Nepa). 



7. As capitate tubes, in the free ends of which many very fine 

 vessels empty themselves ( Tabanus). 



8. As tubes which at intervals are surrounded by spiral caeca 

 ( Cicada). 



9. As granulated glands, which on each side unite into a salivary 

 duct, both of which join into a single excretory duct. Muller has 

 observed this high form of conglomerate salivary glands in Phasma ; 

 Treviranus in Apis; and Burmeister in Locusta, Gryllus, and 

 Termes. 



The biliary secerning organs never attain this condition, but, in all 

 insects, manifest the condition of long^ slender, cylindrical tubes, 

 which Cuvier, who seems not to have been aw^are of the conglomerate 

 structure of certain salivary and seminal glands, describes as the 

 character of all the secreting organs in insects. 



In a few instances, as in Chermes and Aphis, the biliary tubes are 

 wanting: in almost all insects they are four in number, never fewer; 

 sometimes they are six or eight in number : in a few instances, as the 



