338 TirE INFLUENCE OF LIVING SUEEOUNDINGS. 



of their host and of his dwelling. PachyhdeUa lives in preference 

 on the hind part of other crabs, and particularly on the under 

 surface, which, as is well known, the crab always carries folded 

 in under its body towards the front ; it occurs almost exclu- 

 sively on the abdomen of the female crabs. The parasite always 

 (see fig. 12) consists of a somewhat flattened sac, adhering closely 

 on both sides to the surface of the crab's abdomen ; the side 

 edges of the parasite, when seen in its natural position, perfectly 

 agree with the form of the crab's body and correspond exaotly 

 with the lateral symmetry of the crab itself. The structure of 

 one surface of the PachyhdeUa almost always differs from that 

 of the other, and one of these surfaces has hitherto been always 

 regarded as the natural hinder side of the PachyhdeUa and the 

 other as the front or belly ; but this is quite an error, as has 

 been proved by the careful reseai'ches of Professor Kossmann. 

 The old view seemed quite established by the presence of a large 

 opening which was generally recognised as the mouth, and it 

 cannot be denied that a median line of the body seemed to be 

 indicated by this opening, and by the situation, exactly in a 

 line with it, of a style by which the animal attaches itself to 

 the abdomen of the crab, and these allowed of our dividing it 

 into two symmetrical right and left halves analogous to those 

 observed in most other animal forms. But after Kossmann's 

 observations and an exact investigation of its internal anatomy, 

 previously but little known, there can no longer be any doubt 

 that the fiat surface is in fact only one side of the body, and 

 that the two corresponding halves constitute the front and 

 back. Thus, in form, this animal reminds us somewhat of the 

 laterally compressed flat-fishes, in which the back and belly form 

 two edges while the right and left sides are broad and flat ; and 

 like them it always lies on one side, sometimes the right and 

 sometimes the left. 



I cannot resist the temptation to attempt to explain this 

 extraordinary condition of things by an hypothe.iis put forward 

 by Kossmann. The larvse of all the Cirrhipedia — to which 

 PachyhdeUa belongs — are distinct from those of their nearest 

 alUes among the Crustacea by the circumstance that they must 

 pass through a second larva-stage before they can assume the 



