98 



Mr Potts, Observations on the changes 



while the parasite is in its internal stage the change occurs which 

 finds expression in the modification of the secondary sexual 

 characters in the new carapace. If a change of this magnitude 

 can be induced in the early stages of parasitism we might also 

 expect that the effect on growth and moulting would be established 

 and the SaccuUna prevented from ever appearing at the exterior. 

 There does not appear to be any evidence for infrequency of 

 moults in crabs with Sacculina interna. It may here be pointed 

 out that the hermit crab when attacked by Peltogaster* (a near 

 relative of Sacculina) still moults regularly, a fact we can easily 

 trace to the investment of the abdomen by soft skin which tears 

 easily and so detaches itself in moulting, round the base of the 

 parasite. The degree of modification produced in this case is 

 great, but Peltogaster does not mechanically prevent the shedding 

 of the exoskeleton, and growth appears even to be favoured by 

 the presence of the parasite. 



The general nature of change in the secondary sexual characters 

 has already been indicated. The increase in the width of the 

 abdomen is exemplified in the following diagrams similar to those 

 which accompany Giard's paper. It will first be noticed that 



Modified male. 



Female. 



while all the segments of the female abdomen are freely moveable, 

 in the male the third, fourth and fifth are fused together. The 

 first evidence of modification in the infected male is the re- 

 established segmentation of the abdomen. 



The degree of modification may be actually measured by 

 estimating the ratio of the breadth of the abdomen at the segment 



in Fig. ) . The indices so given are as follows : 



to the length , ^^ 



Uninfected male crab '57 — 'oS 



Uninfected female crab *90 



Infected male crab which has suffered the maximum 

 amount of modification '77 



Q. J. Microsc. Sc, Vol. l. 1906, p. 599. 



