THE CIRCULATORY SYSTEM. 37 



Siebold states that in some insects tlie ileum has glandular 

 appendages whose product is perhaps analogous to the pancre- 

 atic fluid. In the larva of insects is found the corpus adiposum, 

 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 by 

 numerous tracheae. 



The Circulatory System. The vascular, or circulatory, 

 sj'stem 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 valvular openings and pumps the blood into 

 its prolongation or cephalic aorta, whence it escapes, 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 empty into two 

 lateral ones, running towards the posterior extremity of the 

 body, 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 ver}^ small, oval, or spheroidal corpuscles, 

 which are always colorless, have a granular aspect, 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 muscles whose apices point outwards. Its walls contain 

 l)oth 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 



