332 



PH YL UM A R THR OPODA . 



true ccelom : It is a cavity which (i) 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 hremoccel. 

 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 

 s.bc a store of reserve material, which is 

 especially large in young insects before 

 metamorphosis, and is also interesting 

 b.c as one of the seats of "phosphor- 



escence.' 



-gn 



n 



Excretory system. Although 

 no structures certainly homo- 

 logous with nephridia have yet 

 been demonstrated in Insects, 



FIG. i6 4 .-Diagrammatic the excretory system is well de- 

 cross-section of an inverte- veloped. From the hind -gut 



(proctodajum), and therefore of 

 ectodermic origin, arise fine 



cc., Ectoderm; s.l>c., reduced f n K pc; nr j n come TflSPS solid 

 coelom;^., gut; b.c., ruemocoel CUUeb > 

 (shaded); e.i:, excretory threads, which extend mtO the 



gSS e or g 1n;T"trainf;ve apparent body cavity. Their 

 s an ha - number varies from two (in some 



Lepidoptera, for instance) to one 



hundred and fifty (in the bee). They twine about the 

 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- 



