REPRODUCTIVE SYSTEM. 



3°9 



true coelom : — It is a cavity which— (1) does not communicate with the 

 vascular system ; (2) does communicate by nephridial pores with the 

 exterior ; (3) has the reproductive elements developed on its lining ; 

 (4) develops either as one or more diverticula from the primitive 

 enteron (or gut), or as a space or spaces in the unsegmented or 

 segmented mesoderm. Now, in Arthropods the apparent body cavity 

 of the adult is not a true ccelom ; it consists of a set of secondarily 

 derived vascular spaces ; it has been called a pseudoccel or a hsemoccel. 

 The true ccelom of Arthropods is very much restricted in the adult, all 

 the more so that most Arthropods (e.g. Insects) have no distinct 

 nephridia. 



But the apparent body cavity in which the organs lie, and in which 

 the blood circulates, is well developed 

 in Insects. It includes, inter alia, a 



peculiar fatty tissue, which seems to be J0*^ <■ ^ m ^fo*^ ec 



a store of reserve material, which is /§r .y^^Sc'^^^- s.bc 

 especially large in young insects before 

 metamorphosis, and is also interesting 



as one of the seats of "phosphor- 8 \Y^~S^rl/J&1l-% — b.c 

 escence. " 



-gn 



Excretory system. — Although 

 no structures certainly homo- 

 logous with nephridia have yet 

 been demonstrated in Insects, 

 the excretory system is well de- 

 veloped. From the hind-gut 

 (proctodeum), and therefore of 

 ectodermic origin, arise fine 

 tubes, or in some cases solid 

 threads, which extend into the 

 apparent body cavity. Their 

 number varies from two (in some 

 Lepidoptera, for instance) to one 

 hundred and fifty (in the bee) 



organs in the abdominal cavity, and their excretory sig 

 nificance is inferred from the fact that they contain uric 

 acid. 



Reproductive system. — Among Insects the sexes are 

 always separate and often different in appearance. The 

 males are more active, smaller, and more brightly coloured 

 than the females. Darwin referred the greater decorative- 

 ness of the males to the sexual selection exercised by the 

 females. The handsomer variations succeeded in courtship 

 better than their rivals. Wallace referred the greater plain- 



FiG. 136. — Diagrammatic 

 cross-section of an Inverte- 

 brate, with a ccelom and 

 a hsemoccel. — After Ziegler. 



ec, Ectoderm ; s.bc, hasmocoel 

 (as in Lamellibranchs) ; ,§-., gut ; 

 b.c, coelom (shaded); ex. , ex- 

 cretory aperture of nephridium ; 

 gn., genital organ ; «., ventral 

 nerve ganglia. 



They twine about the 



