VI ANTENNATA—THE RESPIRATORY ORGANS 477 



The want of a developed arterial vascular system is compensated for as well as 

 conditioned by the extremely profuse branching of the respiratory organs (tracheae), 

 the oxygen being thus conveyed to the blood in all parts of the body. Thus, 

 whilst, as ii rule in the higher animals, the blood in its closed channels generally 

 seeks out the localised seat of the respiratory processes, here, on the contrary, 

 the respiratory organs seek out the blood and the blood tissue in the most remote 

 parts of the body. 



VIII. Fat Bodies — Luminous Bodies. 



In the body cavity lies a mass of large cells filled with small drops 

 of fat, and forming together the so-called fat body. This is variously 

 shaped, and covers inner organs which appear enveloped in it ; it also 

 forms a layer under the integument, etc. It is specially strongly 

 developed in the larvae, and forms a reserve of nutrition, which is 

 dra'ivn upon during metamorphosis, during the formation and ripening 

 of the sexual products, etc. The metabolism which goes on in the 

 fat body is very active, as is proved by the fact that its cells often 

 contain concretions of uric acid. In some cases it has been proved 

 that the fat body in the larva is rich in fat and poor in concretions of 

 uric acid, while in the imago it is poor in fat and rich in concretions 

 of uric acid. 



There are Ooleopfera which possess either on the abdomen {Lampy- 

 riclce) or on the thorax (a few Elatericlw, Pyroplwrus) intensely luminous 

 areas. The seat of light is a luminous organ which, morphologically, 

 must be considered as a specially differentiated portion of the fat body. 

 The cells of this luminous organ secrete, under the control of the 

 nervous system, a substance which is burnt during the appearance of 

 the light ; this combustion takes place by means of the oxygen con- 

 veyed to the cells of the luminous body by the tracheee, which branch 

 profusely in it and break up into capillaries. A weakly luminous 

 dorsal layer of the luminous organ, which lies ventrally in the pen- 

 ultimate and antepenultimate abdominal segments of the Lampyridce, 

 contains very numerous concretions of uric acid. 



Other cell elements occurring in the body cavity {e.g. the pericardial 

 cells lying on the alary muscles of the heart, and occasionally containing 

 fat) cannot here be more closely considered. They form, together 

 Avith the blood corpuscles and the fat body, the so-called blood tissue. 



IX. The Respiratory Organs. 



A. The Tracheal System. 



The respiratory organs of the Antennata are air-eondueting eanals 

 (tracheae) which, on the one hand, communicate with the outer world 

 by means of paired, strictly segmentally arranged outer apertures 

 (stigmata), and on the other spread all over the body, penetrating 



