142 THE APODID^E PART i 



any other Crustacean, and is in itself evidence of 

 the primitive or Annelidan character of Apus. The 

 gradual simplification of the glands from before back- 

 wards towards the less developed segments is also 

 significant. The many-segmented ancestors of Apus 

 developed sexual products in every or nearly every 

 segment. 



In the section on excretion we have already traced 

 back the epithelium forming the sexual glands to the 

 ccelom epithelium of the original Annelid, and to that 

 special part of the epithelium which covered the 

 nephridia. The nephridial tubes themselves have 

 entirely disappeared, having probably been rendered 

 useless by the great size and physiological efficiency 

 of the shell glands. Their peritoneal coverings, how- 

 ever, have remained as the sexual glands. The eggs 

 develop out of this epithelium, not projecting into the 

 lumen of the gland, but outwards, so that they bulge 

 out into the body cavity. This agrees with what 

 takes place in many Polychaetan Annelids ; the eggs 

 develop out of the peritoneum, and apparently out of 

 any part of the same, drop off into the body cavity, 

 and are emptied out through the nephridia. In Apus 

 slight changes have taken place : the eggs develop 

 towards the body cavity out of the peritoneal covering 

 of the vanished nephridia : they do not, however, drop 

 off, but pass through the epithelium again, to pass out 

 through the tube formed by this epithelium, just as 

 they at one time no doubt passed out through the 

 nephridial tubes. When we consider the great size 

 of the eggs owing to the accumulation of the yolk, it 



