SEA-FISHERIES LABORATORY. 297 



Cestode comes to sexual maturity. If, however, other fishes 

 than the definitive final host become infected the life-history 

 of the Cestode cannot be completed, and the larvae develop 

 no further than the Plerocercoid stage, which finally dies and 

 degenerates. But in so doing there is some reaction on the part 

 of the collateral host. That is, there is often some kind of 

 covering laid down by the tissues of the host and this may 

 take extraordinary forms, as for instance, when typical pearl- 

 like bodies are formed round the unrecognisable remains of a 

 parasite of some kind. This is what has, no doubt, happened 

 in the specimens we have. Plerocercoid larvae of tetrarhynchid 

 tapeworms are very common in many teleost fishes, where they 

 appear as little cyst-like bodies attached to the peritoneum. In 

 teleosts the development of a Plerocercoid larva ends the life- 

 history of the Tetrarhynchus, which then dies and degenerates ; 

 but in rays and dogfishes the larva completes its metamorphosis, 

 gets into the intestine and becomes a sexually mature Cestode. 

 Here, then, we doubtless have some Cestode (but evidently 

 not a Tetrarhynchus) which has so entered a cul-de-sac in its 

 life-history, has died, become invested in a cyst wall which 

 shields the host from its further reaction, and so gives rise to 

 these peculiar bodies. 



The "Oyster Parasite," Gasterostomum gracilescens. 



This is the well-known Bucephalus haimeanus, Lacaze- 

 Duthiers.* We have found it already in this district, but 

 never in such a heavy infection as in this case. It occurred in 

 cockles that were being dissected in a vacation Biology course, 

 held at the Piel Laboratory in August, 1921. The infected 

 molluscs were bright yellow in the upper part of the visceral ( 

 mass, and out of 55 specimens examined 3 were infected. The 



* See Miss Lebour, " A Review of the British Marine Cercariae," 

 Parasitology, Vol. IV, No. 4, 1912, p. 424. 



