THE CIRCULATORY SYSTEM. 37 



Siebokl states that in some insects the ileum lias glandular 

 appmuluges whose product is perhaps analogous to the pcmcre- 

 atic fluid. In the larva of insects is found the corjncs adipo.sum, 

 or fat-body, in the form of large lobes of fat-cells which spread 

 through the intervals of the viscera in the general cavity of 

 the body. It is interpenetrated and retained in place l)v 

 numerous tracheae. 



The Circulatory System. The vascuhir, or circulatory, 

 system is not a closed sac as in the Worms and Vertebrates. 

 The organs of circulation consist of a contractile, articulated 

 dorsal vessel, or so-called "heart," which terminates in a 

 cephalic aorta. The dorsal vessel receives the venous current 

 through the lateral valvidar openings and pumps the blood into 

 its prolongation or cephalic aorta, whence it eseai)es, traversing 

 the body in all directions, in regular currents, which do not 

 have, however, vascular walls. "In this way, it penetrates the 

 antennae, the extremities, the wings, and the other appendages 

 of the body, by arterial currents, and is returned by those of a 

 venous nature. All the venous currents em})ty into two 

 lateral ones, running towards the posterior extremity of the 

 bod_y, and which enter, through lateral orifices, the dorsal 

 vessel." (Siebold.) 



"The blood of the Insecta is usually a colorless liquid, 

 though sometimes yellowish, but rarely red. In this liquid are 

 suspended a few very small, oval, or spheroidal corpuscles, 

 which are always colorless, have a granular asi)cct, and are 

 sometimes nucleated. 



"The dorsal vessel, which is constricted at regular intervals, 

 is always situated on the median line of the abdomen, being 

 attached to the dorsal wall of its segments by several trian- 

 gular nuiscles whose apices point outwards. Its walls contain 

 both longitudinal and transverse fibres, and, externally, are 

 covered by a thin peritoneal tunic. Internally, it is lined by 

 another very fine membrane, which, at the points of these con- 

 strictions, forms valvular folds, so that the organ is divided 

 into as many chambers as there are constrictions. Each of 

 these chambers has, at the anterior extremity on each side, a 

 valvular orifice which can be inwardly closed. The returning 



