192 ARTHROPODA [CH. 



thus corresponds in function to the pancreas of the Vertebrata 

 rather than to the liver (see p. 429). What cannot be digested 

 passes out by the perfectly straight intestine which is simply the 

 proctodaeum and is of course lined by cuticle. The alimentary 

 canal is surrounded by a series of spaces which used to be 

 regarded as equivalent to the coelom of the Annelida. This 

 however is not a correct view of their nature. The spaces in 

 question are simply blood spaces. From the study of the develop- 

 ment of the simplest known Arthropod, Peripatus, it is concluded 

 that the coelomic cavities, which in the embryo are just as well 

 represented as they are in Annelida dwindle in size as growth 

 advances and are represented finally merely by the cavities of the 

 kidneys and of the generative organs, whilst the blood spaces enlarge 

 and take on the character of a body-cavity which is then termed 

 a haemocoele in order to distinguish it from the true coelom. In 

 Peripatus there is corresponding to each segment a pair of coelomi- 

 ducts, the so-called " nephridia " which open internally into minute 

 thin-walled sacs, the remnants of the coelomic sacs. In the crayfish 

 however there is but one pair of greatly enlarged "nephridia" which 

 open in front of the mouth by pores situated as already described 

 on the coxopodites of the second antennae. Each of these kidneys 

 consists. of an oval thin-walled sac, developed by an intucking of 

 ectoderm, termed the ureter, in which the excretion accumulates, 

 and by the muscular contraction of which it is expelled, and of a 

 greenish glandular mass situated beneath the ureter, which is the 

 secreting portion of the kidney. This latter portion consists of a net- 

 work of tubes opening into one another, connected on the one hand 

 with the ureter and on the other with a small thin- walled sac the- 

 end sac which is the last trace of the coelomic cavity belonging 

 to this segment. The tubes are lined with excretory cells and since 

 the whole organ is bathed in blood the network arrangement of the 

 tubes permits of the exposure of a large surface to the surrounding 

 fluid from which the excreta are attracted by the excretory cells. 



It must however excite no small surprise that an active and 

 muscular animal like the crayfish should be able to accomplish its 

 excretion by means of a single pair of excretory organs whilst 

 sluggish animals like the Annelida and even Peripatus require a 

 pair of such organs in each segment. 



We are forced to the conclusion that some other organ must 

 assist the "nephridia" in carrying out their function, for there is no 

 question that the active metabolism of the crayfish must produce 



