238 Dr. W. Nlcoll and Mx. W. Small on 



Cercaria e.rcellens, nov. 



This wjis found encysted in large numbers in Carcinua 

 vioenas, and less frequently in Cancer pagurus. It is un- 

 doubtedly the larva of some species of the genus Spelofrema, 

 and, as will be shown later, the adult species to which it can 

 most probably be referred is Sp. e.rcel/ens, Nicoll. 



Three out of every four green crabs were infected ; in the 

 case of the edible crab the infection was not more than one 

 in five. At St. Andrews we have found the infection some- 

 what greater; every green crab examined there contained 

 the cercariae in greater or less numbers, while about 25 per 

 cent, of the edible crabs were infected. In the green crab 

 the number of cercariae is frequently enormous, every organ 

 and tissue in the crab's body being riddled with cysts, so 

 much so that sometimes one would think that the mass of 

 cysts was actually greater than the organ in which they are 

 contained. Tlie chief seat of infection is the liver and next 

 to that the gonads, but no structure is immune, except the 

 calcareous parts. The cysts are occasionally found throughout 

 the muscles and along the course of the nerves, blood-vessels, 

 or alimentary canal. They may occur either singly or in 

 clusters, bound to each other by the fibrous tissue in which 

 they are embedded. 



These observations agree with those of M'Intosh ^, u ho 

 forty years ago found the crabs at St. Andrews infected to 

 the same extent with cercariae. Whether the cercarige which 

 he described then are the same as those we have met with is 

 a matter of doubt, but this will be referred to again later. 



\\ hen extricated from their fibrous investment the cysts 

 are seen to be globular in shape. In some cases they appear 

 to be very slightly ovoid, but this is probably due to pressure 

 in freeing them, and usually they can be made to assume the 

 globular thape by suitable mani|)ulation. At first we were 

 inclined to believe that two different kinds of cysts were 

 present, for many were obviously, even to the naked eye, 

 much smaller than the others. Under the microscope the 

 difference was further accentuated by the fact that the wall 

 of the smaller cysts was proportionately much thinner than 

 that of the large cysts. On more exhaustive examination, 

 however, what may be inter|)reted as intermediate forms were 

 discovered, midway in size between the large and small 

 forms. In point of numbers the large cysts far exceeded the 

 small and intermediate-sized cysts. 



In a series of measurements of about 30 cysts, two-thirds 



* Qrart. .1. Micr. Soc. vol. v. (]86o) p. 201, pi. viii. 



