XV] AND OTHER HOLLOW STRUCTURES 661 



explained the curious form of the egg in Bilharzia (Schistosotna) 

 haematobium, a formidable parasitic worm to which is due a disease 

 wide-spread in Africa and Arabia, and an especial scourge of the 

 Mecca pilgrims. The egg in this worm is provided at one end 

 with a httle spine, which now and then is found to be placed not 

 terminally but laterally or ventrally, and which when so placed 

 has been looked upon as the mark of a supposed new species, 

 S. Mansoni. As Looss has now shewn, the little spine must be 

 explained as having been moulded within a little funnel-shaped 

 expansion of the uterus, just where it communicates with the 

 common duct leading from the ovary and yolk-gland; by the 

 accumulation of eggs in the ootype, the one last formed is crowded 

 into a sideways position, and then, where the side-wall of the egg 

 bulges in the funnel-shaped orifice of the duct, a little lateral 

 "spine" is formed. In another species, S. japonicum, the egg is 

 described as bulging into a so-called "calotte," or bubble-like 

 convexity at the end opposite to the spine. This, I think, may, 

 with very little doubt, be ascribed to hardening of the egg-shell 

 having taken place just at the period when partial relief from 

 pressure was being experienced by the egg in the neighbourhood 

 of the dilated orifice of the oviduct. 



This case of Bilharzia is not, from our present point of view^ a 

 very important one, but nevertheless it is interesting. It ascribes 

 to a mechanical cause a curious pecuharity of form ; it shews, by 

 reference to this mechanical principle, that two conditions which 

 were very different to the systematic naturalist's eye, were really 

 only two simple mechanical modifications of the same thing; 

 and it destroys the chief evidence for the existence of a supposed 

 new species of worm, a continued behef in which, among worms 

 of such great pathogenic importance, might lead to gravely 

 erroneous pathological deductions. 



On the Form of Sea-urchins 



As a corollary to the problem of the bird's egg, we may consider 

 for a moment the forms assumed by the shells of the sea-urchins. 

 These latter are commonly divided into two classes, the Regular 

 and the Irregular Echinids. The regular sea-urchins, save in 



