232 



by mutual pressure and fitting into each other, so to speak, 

 in the way shown in the section. 



The outer wall of the whole cyst is fibrous in structure 

 with a thin, external serous layer, and among the fibrous layers 

 there are a few blood vessels. This part, therefore, belongs to 

 the host. The fibrous layers send inwards a few trabeculae 

 between the secondary cysts, but apart from this there are no 

 other tissues. The mternal, secondary cysts have walls of quite 

 a different nature, these consisting of an apparently homo- 

 geneous substance (it is represented by the thick black bands 

 in Fig. 3. It may be called " elastic tissue," though it is not 

 exactly like this. It is yellow in colour. Within these 

 secondary cysts there is nothmg but an mirecognisable cell 

 debris containing much oil, which tends to coalesce in droplets 

 when the cyst is opened and scraped out. No remains that 

 can be ascribed to known parasites can be identified — no hooks, 

 spines, or calcareous corpuscles. This is rather a difficulty in 

 assuming that we have to deal with a degenerate cestode larvae, 

 for the hooks and spines are very persistent. Still, the parasite 

 may not have had any skeletal structures and the calcareous 

 corpuscles may have been dissolved for the specimen had been 

 kept for nearly two years in spirit and part of it was fixed in an 

 acid preservative. 



A taxidermist who had the fish supposed that the cysts 

 were ova that had been eaten by the fish, but this is obviously 

 not the case. Nor are they ova that have not been extruded 

 for there is no trace of egg structure. It is exceedmgly likely 

 that they are really small groups of Plerocercoid larvae which 

 have degenerated. In many fishes there are collateral life- 

 histories for the contamed Cestode and Trematode parasites : 

 that is, there is an infection arising by the fish eating some 

 animal which contains the larvae of the Cestode. Now the 

 latter has usually " definitive " hosts, one which contams the 

 larvae and which is eaten by the definitive final host, where the 



