518 



Mr, W. NicoU on sotne 



which I liave already described * from Cardium edule, 

 Mytilits eduh's, and Mactra stultorum. I have not been able 

 to prove this as Villot did, in the case of Cercaria hptosotna, 

 by observing the development of the cercaria into the adult 

 within the intesline of the host; but comparison of the adult 

 with the encysted cercaria? in the cockle brings out several 

 points of resemblance. 1"he most marked is the close agree- 

 ment in the number and disposition of the cephalic spines. 

 In loth the number is 29, and they are arranged round the 

 oral sucker in a single row, uninterrupted, save in the mid- 

 ventral line. Tiie peculiar arrangement of the spines at each 

 end, where two are on a different level from the rest and 

 much smaller in size, is the same in both. The shape and 

 position of the suckers also correspond, allowance being made 

 for the great backward elongation of the body in the adult. 

 The initial stages of this transformation I have observed 

 within the in\estme of Ucematopusostralegus, in whicli several 

 cercariae which had only recently emerged from the cyst were 

 found. They differed from the encysted cercarise in the 

 cockle in no respect save the increased length of the body. 

 Comparison of the measurements of these with corresponding 

 measurements in the encysted cercaria and in the adult found 

 in Larus ridihundus demonstrates the fact that the growth 

 of the suckers compared with that of the length of the body 

 takes place in the same proportion from the first to the second 

 stage as from the second to the third. This will be best seen 

 from the following table : — 



Encysted 



cercaria in 



cockle. 



Length 77 mm. 



Oral sucker (diameter) "OGo „ 



Ventral sucker „ '095 ,, 



Increase in lenglli 



„ oral sucker 



„ Tentral sucker .... 



Eatio of increase in oral sucker 



to increase in length .... 



Ditto, ventral sucker 



From these considerations it seems probable that Echino- 

 stomum secundum is the adult form of the encysted cercarisB 

 of the cockle and the mussel f. 



* Ann. & Mag. Nat. Hist. ser. 7, xvii. p. 148, pi. iv. fig. 5. 



t Later I found E. secundum occuning very numerously in Lanis 

 argentattis. The examples in this case were in an earlier stage of deve- 

 lopment, being smaller than those in L. rulibmidi(s and containing few or 



