72 ANIMAL PARASITES. 



lengths, and composed of several coats; longer in herbivorous 

 than in carnivorous species,, without a peritoneum,, and only hejd 

 in its place by the ramifications of the air-passages. We find a 

 mouth, a muscular oesophagus, on which is a pharynx which is 

 usually folded and furnished with racemose or tubular salivary 

 glands, then around gizzard with horny ridges, bristles, and teeth; 

 and, in sucking insects, also a sucking stomach hanging by a stalk 

 near the oesophagus. Upon the gizzard follows the very large 

 lobate, folded chylific stomach, which is beset, to a greater or less 

 extent, with short glandules (villi), and into the hinder end of 

 which the kidneys open ; these are urinary vessels, either csecal, 

 or passing into each other to form loops, and containing yellowish 

 or reddish urine, which embrace the intestine. The intestine 

 presents a narrow ileus, and a cloaciform large intestine, clothed 

 internally with transparent ridges of glands, and sometimes with 

 a caecum and a muscular rectum. The anus is situated in the 

 last segment, sometimes with anal glands with stinking or poison 

 contents, which also occur at the articulations of the legs (Meloe), 

 or on the lower surface of the chest (bugs). Connected with the 

 alimentation are the fat-glands, which are retrograde in the mature 

 insect. 



The circulatory system exhibits a tubular heart or dorsal vessel, 

 composed of several usually eight chambers, furnished with 

 (sixteen) lateral openings and valves, which acts as a syringe, and 

 a main artery, passing through the thorax and head, and termi- 

 nating freely in one or several openings, from which the colour- 

 less blood, containing only a few colourless corpuscles, is distributed 

 through wall-less canals in various directions through the body. 



The respiratory organ is a system of ramified tracheae, or air- 

 tubes, running in all directions through the body, even into the 

 feet, &c., which communicate with the outer world by peculiar 

 openings, spiracles or stigmata, placed in pairs on the sides of the 

 body, often differently coloured, surrounded by peculiar horny 

 rings, and capable of opening and closing by the agency of horny 

 rings. They carry the air to the freely circulating blood. They 

 form membranous, double-walled, multifariously branched tubes, 

 which are kept open by a spiral horny filament laid between them, 

 and which is only wanting in the smallest ramifications ; in flying 

 insects they are often dilated into air-sacs (without spiral fila- 

 ments). These tracheae either present two large wide stems on 

 each side of the ventral cord, into which the stigmata open and 



