336 ANIMAL PARASITES. 



heart, and once even to a valve of the dorsal vessel, so that the 

 little animal was shaken to and fro with the valve, and in the 

 same way Leuckart, according to communications by letter, has 

 also found the brood of the cestode worms so abundantly m 

 blood-vessels that he thinks the blood is the most usual course of 

 migration, and furnishes the readiest explanation of the widely 

 diffused occurrence of analogous grades of development of other 

 parasites in the same body. For my own part, I believe that 

 both courses of migration are followed simultaneously, but I do 

 not venture to give an opinion as to the greater frequency of one 

 or the other. With regard to the possibility of a self-infection 

 in the case of those who harbour TricJwcephalus dispar, the 

 reader may compare what has been said respecting self-infection 

 with Cysticercus celluloses. But however this may be, 

 after the immigration, which may certainly often occur in 

 this manner that a second, third, &c, embryo follows the 

 line of march struck out by its predecessor and remains 

 stationary behind it (linear arrangement) or in its neighbourhood 

 (arrangement in groups), the further process is certainly the 

 following, with which we have already become acquainted amongst 

 the Ceslodea, and which Meissner has distinctly traced in the 

 larva? of Ephemera. When the embryos of the Gordiacea reach 

 a spot which the} r recognise as suitable, they resign themselves to 

 repose, lie still, contract their head and proboscis, bend the 

 abdomen again sharply round, so that the caudal extremity comes 

 to lie close to the anterior extremity, without presenting any new 

 organ, or throwing off any other, and then surround themselves 

 closely with a clear thin wall, sharply limited externally and 

 produced by secretion from the Gordius itself, whilst the muscular 

 fasciculus surrounding it loses its structure, and a granular, 

 fragmentary mass lies around it, frequently also indicating the way 

 by which the Gordius came ; or in other parts of the body, the 

 host forms a cyst closely adherent to his tissues, consisting 

 of concentric layers of a fibrous or lamellar substance with 

 imbedded cell-nuclei. The processes in the encystation of the 

 Trichina may take place in this last-mentioned manner. The 

 brood, when it has arrived at the desired place, comes to a 

 state of repose, approximates the head and caudal extremity in 

 spiral turns, without however contracting a part of the head, and 

 is then perhaps surrounded even on the part of the worm with a 

 peculiar layer, but certainly enveloped, on the part of the host, 



