308 PERIPATUS, MYRIOPODS, AND INSECTS. 



within the rectum, in and out of which water flows. In either case, 

 an interchange of gases between the trachea; and the water takes place. 

 In adult aerial life the trachea? 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 the case. 



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

 parasitic larvae of some flies, e.g. gadflies. 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 



g—^-n » pvJ/~1 — ex not k e highly developed. 



Within a dorsal part of the 

 body cavity, known as the peri- 

 cardium, the heart lies, swayed 

 by special muscles. It is a 



Fig. ^.-Diagrammatic cross- lon g tube > usually confined to 

 section of an Invertebrate, with the abdomen, usually of eight 



the cceiom (be), shaded. -After chambers, with paired valvular 

 !f g ",' ,,,.,, , t • openings on its sides, through 



c c. , Ectoderm ; ol. , bladder of nephri- 1 ■ 1 11 j r . i 



dium (as in Crustaceans); ex., ex- Which DlOOd enters from the 



cretory ^ duct ; gn., genital organ; pericardium. The blood is 



«., ventral nerve-cord ; e., gut;/., \. . 



funnels of nephridia (as in worms). 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 circulates in spaces within 

 what is commonly called the body cavity. Such a circula- 

 tion 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. — It is necessary to distinguish the primitive cceiom 

 from the apparent body cavity of the adult. In discussing the develop- 

 ment of Peripatus, Sedgwick notes the following characteristics of a 



