240 Dr. ^^\ Kicoll and Mr. W. Small on 



due allowance being made for the stage of development. 

 The dimensions of the chief organs have already been given. 

 The ventral sufker is situated at the beginning of the 

 posterior third of the body length. The genital sucker lie.s 

 close to its left side. The intestinal diverticula terminate at 

 the level of the centre of the ventral sucker. The testes lie 

 close behind that sucker, one on each side, but the yolk- 

 glands are not visible. In most cases the ovary can be made 

 out on the right side of the ventral sucker, and in some the 

 vesicula seniinalis in front of the sucker. 



As already mentioned, this cercaria is probably the larva of 

 a Spelotreina species. The only other genus wiiich might 

 come into question is Levinsenidla, l)ut the character of the 

 genital sucker points rather to Speloirema. Of the species of 

 the latter genus the only one which can be considered is 

 SpeJ. ea'cellens, Nicoll,the large size of the cercaria excluding 

 the possibility of its identification with any of the other 

 species of the genus. 



The adult S/x'hdrema e.rceJIens is found in great abundance 

 in the herring-gull {Larus arfjeniatns), both at St. Andrews 

 and Millport. S/ielotrema simile, Jagersk., the next largest 

 Spdotrema species, is much rarer. 



With regard to the cercaria described by M'Intosh *, it is 

 unfortunate that he gives no exact measurements either of 

 the cysts or of the cercarite. By measurement of his figures 

 (pi. viii.), which are magnified Ib'O diameters, we find that 

 the cyst in fig. 1 had a diameter of •14 mm., and that in 

 fig. 2, "17 mm. The cercaria in fig. 5 is exactly '5 mm. long. 

 It requires but a glance at the plate to see that the cercaria 

 in fig. 5 is much too large to have come from a cyst of the size 

 shown in figs. 1 and 2. By the kindness of Professor ^M'lntosh 

 a tube containing some of his original material from Carcinus 

 mwiias was placed at our disposal, but in it only one cyst 

 was found. The diameter of this was '29 mm. ; the outer 

 radially-striated wall was "Ol mm. thick and the inner con- 

 centrically striated '012 mm. From these somewhat meagre 

 details we are forced to the conclusion that Professor M'Intosh 

 also had a variety of cysts under his observation, some of 

 which correspond in size with those which we have found, 

 but others being much smaller. The only other explanation 

 is that some error has crept into his estimation of the magni- 

 fication. It is hardly to be believed that the character of the 

 infection of the crabs at St. AndrcAvs has undergone a change 

 from small cysts to large ones during the space of forty 

 years. 



* Loc. cit. 



