348 CHARACTERS OF INVERTEBRATE ANIMALS 



into a small mouth-cavity, from the back of which projects a 

 pointed tongue, and which passes into a narrow gullet continuous 

 with a large thin-walled crop, and that again with a smaller thick- 

 walled gizzard, from which a fairly long intestine runs to its 

 termination between the cerci. The gizzard is provided with 

 an internal chewing arrangement, such as is not uncommon 

 among Arthropods. It consists of a firm lining raised into 

 horny teeth and bristly ridges. Digestive fluids are poured 

 into the digestive tube by a pair of branching salivary glands 

 which open into the mouth-cavity, and a number of club-shaped 

 ''liver" tubes (digestive caeca) which encircle the beginning of the 

 intestine. 



Circulatory Organs (fig. 202). — This is not very highly special- 

 ized, for a reason to be mentioned later. The whole of the body 

 is traversed by irregular blood-spaces of various sizes, but the 

 only definite tubular structure is the heart, a long narrow blood- 

 vessel running through the thorax and abdomen close to the 

 upper surface. Along its sides are numerous pairs of valvular 

 openings, through which blood passes in from the surrounding 

 blood-space (pericardial sinus) to be pumped forwards. The heart 

 is systemic, as it contains pure blood. 



Respiratory Organs (fig. 202). — These organs present a very 

 interesting and remarkable arrangement, for instead of the impure 

 blood being sent to a localized lung for purification, air is con- 

 veyed to all parts of the body by means of branching tracheal 

 tubes. These communicate with the exterior by means of ten 

 pairs of small openings or stigmata on the sides of the body, 

 all but the first two pairs being in the abdomen. Observation 

 of a living Cockroach will show that the abdomen alternately 

 dilates and contracts, the result being that air is drawn into and 

 expelled from the stigmata. It is therefore possible to choke 

 an insect by smearing the sides of the body with oil or some 

 other substance which will block up these openings. The tracheal 

 tubes appear silvery under the microscope, owing to the air which 

 they contain, and they are lined by an elastic membrane thickened 

 into a continuous spiral thread, so as to render them very flexible 

 and non-collapsible. Just within each spiracle is a kind of valve, 

 formed as a projection of the lining membrane and helping to 

 prevent the intrusion of foreign particles. 



As a result of this very thorough system of aeration, the blood 



