944 ON THE SHAPES OF EGGS [ch. 



spine may be at the end or towards the side of the egg : and this 

 -visible difference has led to the recognition of a new species, S. 

 mansoni*. In a third 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 httle 

 doubt, be ascribed to hardening of the egg-shell having taken place 

 just at the time 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 pecuUarity of form ; and it shews, by 

 reference to this mechanical principle, how two simple mechanical 

 modifications of the same thing may not only seem very different 

 to the systematic naturaUst's eye, but may actually lead to the 

 recognition of a new species, with its own geographical distribution, 

 and its own pathogenic characteristics. 



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 Regujar 

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

 details which do not affect our problem, have a complete axial 

 symmetry. The axis of the animal's body is vertical, with mouth 

 below and the intestinal outlet above; and around this axis the 

 shell is built as a symmetrical system. It follows that in horizontal 

 section the shell is everywhere circular, and we need only consider 

 its form as seen in vertical section or projection. The irregular 

 urchins (very inaccurately so-called) have the anal extremity of 

 the body removed from its central, dorsal situation; and it follows 

 that they have now a single plane of symmetry, about which the 

 organism, shell and all, is bilaterally symmetrical. We need not 

 concern ourselves in detail with the shapes of their shells, which 

 may be very simply interpreted, by the help of radial coordinates, 

 as deformations of the circular or "tegular" type. 



* L. W. Sambon, Proc. Zool. Soc. 1907 (i), p. 283; also in Journ. Trop. Med. and 

 Hygiene, Sept. 15, 1926. 



