CIRCULATORY SYSTEM. 315 



breathe by " tracheal gills " little wing-like outgrowths from the sides 

 of the abdomen, rich in tracheae or by tracheal folds within the rectum, 

 in and out of which water flows. In either case, an interchange of gases 

 between the tracheae and the water takes place. In adult aerial life, 

 the tracheae of the body acquire stigmata, and the insect becomes 

 " holopneustic." 



In most insects with complete metamorphosis, the larva (e.g., cater- 

 pillar or grub) has closed stigmata on the last two segments of the 

 thorax (those which will bear wings), but there is a pair of open stigmata 

 on the prothorax. In the adult the reverse is true. 



There are some other modifications, for instance what obtains in the 

 parasitic larvae of some flies, e.g., gad flies. In these the stigmata are 

 open only at the end of the body. In all cases, however, the stigmata 

 of the adult are already present as rudiments in the larva, though they 

 may not open till adolescence is over. 



Circulatory System. 



As the respiratory system is very efficient, establishing 

 the possibility of gaseous interchange between the inmost 

 recesses of the body and the external medium, it is natural 

 that the blood vascular system should not be highly 

 developed. Within a dorsal part of the body cavity, known 

 as the pericardium, the heart lies, swayed by special muscles. 

 It is a long tube, usually confined to the abdomen, usually 

 of eight chambers, with paired valvular openings on its 

 sides, through which blood enters from the pericardium. 

 The blood is driven forwards, the posterior end of the 

 heart being closed, and there is usually an anterior aorta or 

 main blood vessel. But, for the most part, the blood cir- 

 culates in spaces within what is commonly called the body 

 cavity. Such a circulation is often described as lacunar. 

 The blood may be colourless, yellow, red, or even greenish, 

 and, in some cases, haemoglobin, the characteristic blood 

 pigment of Vertebrates, has been detected. The cells of 

 the blood are amoeboid. 



Body Cavity. 



One is apt to -use this term in two senses for the primitive body 

 cavity or ccelome, and for the apparent body cavity of the adult. In 

 discussing the development of Peripatus, Sedgwick notes the following 

 characteristics of a true ccelome : It is a cavity which ( I ) does not 

 communicate with the vascular system ; (2) does communicate by neph- 

 ridial pores with the exterior ; (3) has the reproductive elements developed 

 on its lining ; (4) develops either as one or more diverticula from the 



