Larval Trematodes. 239 



were found to liave a diameter between '43 mm. and '49 mm. 

 In these the thickness of the outer wall was '027-*05i mm., 

 average '036 mm. ; and the thickness of the inner wall 

 •008-"023 mm., average -017 mm. Thus the outer wall is 

 two or three times as thick as the inner, but in one or twj 

 cases the ratio was not more than 3 : 2. Onlj in a few 

 cases was the cercaria expressed from the cyst in an undama;^ed 

 condition. The approximate length of the cercaria was found 

 to be 'S-l'O mm. In every case the oral sucker was larger 

 than the ventral, the diameter being •06f)-*03 mm. for the 

 oral and "OG-'OGS mm. for the ventral. In some cases the 

 genital sucker was also measured and found to have an 

 average diameter of '052 mm. 



Of the remaining cysts a group of four measured '38- 

 •39 mm., average '37 mm. In these the outer wall had a 

 mean thickness of "Olo mm. and the inner '006 mm. In 

 only one case was the cercaria obtained in an undamaged 

 state and its length was *7 mm. No difference in size could 

 be detected between the oral and ventral suckers, each 

 measuring "054 mm., while the genital body measured 

 •048 mm. 



A third group of five much smaller cysts measured '27- 

 •32 mm., average "305 mm. The outer and inner walls were 

 •Oil mm. and '007 mm. thick respectively. None of the 

 cercariaj from these were examined. From these figures 

 there seems no reason to suppose that these groups are other 

 than stages in the growth of the same cyst, and such being 

 the case it is evident that the cercariae increase considerably 

 in size during their sojourn in the crab. The only alternative 

 is that they represent the larvse of three different adult 

 species. It is unfortunate that more detailed examination of 

 the smaller cercaiiae was found impossible, as that might 

 have been of help in deciding the matter. 



A fourth variety of cyst was met with, but only on one or 

 two occasions. It was about the same size as the foreg-oinff, 

 but differed in having a cell-wall composed of only one very 

 thin ('002 mm.) layer. In none of these was the cercaria 

 examined. They must apparently belong to a distinct species, 

 unless, indeed, they are abnormalities. A possibility wiiich 

 might be suggested is that they are just some of the ordinary 

 cysts in which the outer layer has been accidentally stripped 

 off; but there was no evidence of this and in addition the 

 thickness of the wall is much less than that of the inner layer 

 even in the smallest of the cysts. 



A detailed description of the anatomy of the cercaria is 

 not necessary here, as it agrees well with that of the adult, 



17* 



